Book - Outline of Comparative Embryology 2-9

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Richards A Outline of Comparative Embryology. (1931)
1931 Richards: Part One General Embryology 1 Historical Development of Embryology | 2 The Germ-Cell Cycle | 3 Egg and Cleavage Types | 4 Holoblastic Types of Cleavage | 5 Meroblastic Types of Cleavage | 6 Types of Blastulae | 7 Endoderm Formation | 8 Mesoderm Formation | 9 Types of Invertebrate Larvae | 10 Formation of the Mammalian Embryo | 11 Egg and Embryonic Membranes | Part Two Embryological Problems 1 The Origin And Development Of Germ Cells | 2 Germ-Layer Theory | 3 The Recapitulation Theory | 4 Asexual Reproduction | 5 Parthenogenesis | 6 Paedogenesis And Neoteny | 7 Polyembryony | 8 The Determination Problem | 9 Ecological Control Of Invertebrate Larval Types

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This historic 1931 embryology textbook by Richards was designed as an introduction to the topic. Currently only the text has been made available online, figures will be added at a later date. My thanks to the Internet Archive for making the original scanned book available.
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Chapter IX Ecological Control Of Invertebrate Larval Types

Throughout the animal kingdom many diverse environmental relations surround the eggs and young so that many responses might be catalogued. Environmental influences find expression in the places where the eggs are laid, in the amount and kind of food provided for the young during the early development, in the different devices for their protection, in specialized modifications for accomplishing locomotion and other vital activities, and in many cases in the production of forms of larvae ‘which are totally different from the adult organisms. These special types of larvae present a great range of variations from the modes of direct development with which the student of the embryology of vertebrates is familiar. These larval types involve a metamorphosis to the adult form which may be more or less complete and which in such cases brings the growing organism into an environment quite unlike that of its earlier development; metamorphosis is necessary in those forms in which the food and habits of the adult are unsuitable for developing young. If we assume the sea to have been the original home of most primitive stocks, an assumption which is more or less common and seems in line with the fact that very many types of animals develop in a moist atmosphere or liquid medium, we should expect the majority of unusual larvae to be found in the species living in salt water. The transition from salt water to fresh or to the terrestrial forms of life has presented many difficulties to the developing young and as a result those animals which live in fresh water or are terrestrial are more often characterized by direct development than are the salt-water forms. It may be regarded as an axiom of development that sessile adults have active young. This activity has two obvious results; first, the species secures its dispersal through the migration of the young, and econd, it extends the range. The migration is commonly passive but over a period of time serves to extend the range of the species as successfully as if the adults themselves were able to move. To insure dispersal, vast numbers of gametes must be produced, for the chance method of fertilization that usually accompanies such cases results in a failure of very many eggs and sperm to become further activated, and of those eggs which are fertilized only a few can come to maturity. The larval mortality is very high indeed, for these young are the food of many species as well as the victims of physical forces and of their own inability to find continuously satisfactory conditions for growth. The vast number of eggs produced may be realized by reference to the turbot which produces 9,000,000 in a single season, the cod with 5,000,000, and to the flounder with 1,000,000. In higher animals the reproductive energy is conserved by various means for caring for the young during early life, but in marine forms this is usually not the case. The second consequence of larval activity is to bring the growing forms into an entirely difierent range. This is illustrated in the case of the lobster, which is a bottom feeder, lurking in the crevices between stones and elsewhere to capture whatever prey may come within its reach, or feeding upon such dead forms as may be found nearby. The larvae, up to the time of the fourth molt, however, swim at the surface of the sea water. In addition to this, they are positively phototropic to light of the intensity of ordinary daylight, but the adults are negative. Thus the pelagic larvae are brought into a range of environment where their food, which consists of plankton, especially copepods, is abundant. Special structural modifications adapt these larvae to their surface pelagic life. They are the better enabled to swim on the surface of the water because of the possession of exopods, the outer branches of the walking legs characteristic of lower crustaceans but not present in the adults of the higher forms. These are retained to the fourth molt, when they are cast off, being no longer useful; the larva then goes to the bottom. Herrick makes note of several changes in structure and instincts which take place at the beginning of the fourth stage, which marks the most surprising leap in the whole history of development. Among these are the following: the primitive swimming branches of the thoracic appendages are lost; the cuticle becomes shell— like, containing more lime; the pigments are denser, the colors brilliant, and the color pattern variable; otocysts are present and orientation is perfect; rotation of the great forceps is complete; the animal, during at least a part of this stage, moves toward the light and swims steadily at the surface with the great claws directed forward and held close together; the preying instinct is more marked; the fighting instinct, the instinct of fear, “feigning,” and hiding are all developed by the close of the fourth stage or in the fifth, when the animal goes to the bottom to stay. In many other animals similar gain is accomplished by a specialized form of larval structure. The larva finds itself adapted to-securing food which is suitable for it, and structures which have only temporary employment are present and useful. It follows, however, that the gain to the larva would become a loss if the subsequent stages in the life cycle were not radically changed in such a way as to enable the organism to undertake a new mode of life. In other words, metamorphosis in such cases is a necessity for bringing about those adaptations which fit the organism to live in its permanent environment and

there to undergo those further changes which look toward the production of new germ cells.

It is not usual to find metamorphosis in animals which live in fresh water, although there are some outstanding cases in which larvae totally unlike the adult are developed in species inhabiting fresh water. It may, however, be taken as a general rule that development in freshwater forms with the exception of certain few highly specialized cases is direct. Fresh water presents a great lack of constancy in living conditions as compared with those of the sea. With but few exceptions the bodies of fresh water lack sufficient depth and area to maintain even a low degree of constancy. Fresh water becomes heated much more easily than the sea, it freezes comparatively quickly, its streams are subjected to periods of flood during which the water runs rapidly and scours out the beds or overflows, and when it recedes leaves innumerable forms of life stranded to perish. Every stream has narrows and depths where it runs rapidly and flats where its current is slow, an environment in which the fragile larvae would scarcely be able to maintain themselves, and there are but few animals which go through an independent larval history in fresh water. The lack of suitability of fresh water to larval development without doubt explains why so many groups of marine animals have not been able to gain a foothold in fresh water. A very few sponges, almost no coelenterates, occur in this environment, and many groups of marine fish and ascidians, cephalopods, king crabs, and some of the worm groups have not a single representative in fresh water. Animals that cannot produce eggs which develop into young like the adult are prevented from gaining ‘a. foothold by such a changeable medium as fresh water. In addition to the difficulties of fresh-water life already noted, a zone of brackish water between the fresh water and the sea is itself an almost impassable barrier to the entrance of marine forms.

Terrestrial life presents to developing larvae even more dangers than fresh water. To this situation is due the fact that in land forms development is either direct and fairly simple or else specialized with complicated devices for caring for the young during their immature period. The latter condition is illustrated by many forms of insects in which metamorphosis is complete, often involving very complicated life histories. Some of the disadvantageous features of the terrestrial 386 ECOLOGICAL CONTROL OF INVERTEBRATE LARVAL TYPES

environment are the following: temperature undergoes a wide range of variation, often passing quickly between the extremes. This condition is quite unknown in the ocean and is much less in fresh water. The very changeableness as well as the extremes of heat and cold are hardships for the developing larvae. The weight of the body is no longer buoyed up by water and must be supported at every moment. This involves consumption of much energy as compared with the almost passive drifting of larvae in aquatic habitats, and renders movement much more difficult and restricted. Food no longer streams by as in the contrasting case, but must be actively sought, is often very much more restricted as to kind, and presents an increased toughness since in the composition of land forms the percentage of water is much less, and the food is necessarily more difficult to find. The glare of daylight and the consequent difficulty of avoiding enemies puts the terrestrial larvae to a disadvantage which forms living in the subdued shadows of the water do not share. finally the rapid evaporation to which terrestrial forms are subjected requires special devices for protecting the soft exterior of the organism. A heavy shell, mucous glands, and other similar devices show what a serious drain upon the organism requirements of this kind cause.

These considerations account in a large measure for the lack of uniformity in the occurrence of larval types throughout the different groups of the animal kingdom. The particular forms of larvae within a group are of adaptive rather than taxonomic significance.

The general subject of the care of young animals was briefly but instructively discussed by Gamble in “The Animal World” (Holt and Company). Some of the ideas mentioned on the three preceding pages are suggested by his discussion. Gamble sums up the responses of animals to different environmental conditions in which the young must be produced as follows:* “We find isolated examples of the retention of the young by diminution in the size of the family. Speaking generally, marine animals pour their eggs broadcast, and leave their minute larvae to complete their metamorphosis unaided and unsheltered, but in every group there are malthusian species which not only restrict their families in number, but enclose them by protective envelopes. The varied experience of larval life in this curtailed and direct development supplants metamorphosis. The eggs become larger and the young stronger at birth.

“In fresh water the limitation and protection of the family is more generally the rule. Insects and Amphibia are the only large classes

‘ Quoted by permission of Henry Holt & Co.

which go through their larval life in fresh water. But the protection

is usually of the simplest kind and is confined to the earlier stages of development.

“On land the insects form the only class in which the majority still pursues a free larval history. Others develop directly, either growing up into full stature without guidance or protection, or carried and fed by their parents for some time both before and after hatching.”

Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

1931 Richards: Part One General Embryology 1 Historical Development of Embryology | 2 The Germ-Cell Cycle | 3 Egg and Cleavage Types | 4 Holoblastic Types of Cleavage | 5 Meroblastic Types of Cleavage | 6 Types of Blastulae | 7 Endoderm Formation | 8 Mesoderm Formation | 9 Types of Invertebrate Larvae | 10 Formation of the Mammalian Embryo | 11 Egg and Embryonic Membranes | Part Two Embryological Problems 1 The Origin And Development Of Germ Cells | 2 Germ-Layer Theory | 3 The Recapitulation Theory | 4 Asexual Reproduction | 5 Parthenogenesis | 6 Paedogenesis And Neoteny | 7 Polyembryony | 8 The Determination Problem | 9 Ecological Control Of Invertebrate Larval Types


Cite this page: Hill, M.A. (2020, October 25) Embryology Book - Outline of Comparative Embryology 2-9. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Outline_of_Comparative_Embryology_2-9

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