Book - The Pineal Organ (1940) 13
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Gladstone RJ. and Wakeley C. The Pineal Organ. (1940) Bailliere, Tindall & Cox, London. PDF
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Chapter 13 Eyes of Intermediate Types Between Invertebrates and Vertebrates
These, commonly known as Protochordata, are classed in three Subphyla :
- Hemichorda, including Balanoglossus, Fig. 123.
- Urochorda, comprising the tunicates (Ascidia or sea-squirts) ; Salpa (Fig. 128, A, Chap. 14, p. 176) ; Doliolum and allied forms, many of which are fixed and form colonies such as Pyrosoma.
- Euchorda (Acrania or Cephalochorda), represented by Amphioxus or lancelet (Fig. 129, Chap. 16, p. 179).
All these classes are interesting with respect to the eyes which are present in the larval stages of certain representative types, and from the general standpoint of morphology although they are not in the main line of descent of vertebrates and are to be regarded as side-branches some of which have undergone regressive changes.
There is still considerable difference of opinion with regard to the affinities of each of the three groups enumerated above, which are sometimes described together under the general term Protochordata. The disagreement has been specially concerned with the relationships of the Subphylum Enteropneusta, or Hemichorda, which includes such apparently different species, as the free-swimming Balanoglossus and the fixed type Cephalodiscus. The affinity of these with the vertebrates was at one time definitely denied by many morphologists, although the Urochorda or tunicates have for a long time been recognized as degenerate vertebrates which have undergone adaptational changes, such as a colonial or a fixed condition of life in the adult animal which is similar to that of many protozoa or coelenterates and aquatic plants.
The very close resemblance of the tornaria larvae of the Hemichorda (Fig. 125, Chap. 14, p. 172) to the trochophore larvae of invertebrates, more particularly of the annelids and the echinoderms — a resemblance which includes such likenesses as are found in the apical plate, the cerebral ganglia, the two ocelli ; the disposition of the ciliated bands and apical tuft of hairs (Fig. 124) ; also of the mouth, alimentary canal, and anus — led certain authors such as Spengel to challenge the earlier conclusions of Bateson and others that they were derived from and represented the parent stock, which had given rise to the whole vertebrate kingdom. Later workers, including Ritter and MacBride, have, however, re-affirmed an important item in the original thesis of Bateson, namely that bilateral segmented animals have evolved in two directions — one, in which the alimentary canal pierces the nervous system between the supra-oesophageal ganglion and the infra-oesophageal ganglion, leading up to the annelids and arthropods ; and the other, in which the central nervous system is dorsal to the alimentary canal, from which the protochordata and vertebrates have been evolved. One important distinguishing character of the Hemichorda may be noted, which serves to connect them with the vertebrate stock, namely, the presence of a series of paired gill slits behind the " collar region," which communicate with the pharyngeal region of the alimentary canal (Fig. 123, p. 169). Somewhat similar gill slits are also present in larval tunicates, which combined with a ventral position of the heart and other definitely vertebral relations of the intestinal organs indicate that these lowly organized animals, the Protochordata, are sidebranches of the early vertebrate stem ; but it must not be concluded that they are in the direct line of descent, or, in other words, belong to the true parental stock of the vertebrates . The intermediate position of Balanoglossus is indicated by the co-existence in it of both dorsal and ventral nerve cords, as well as the larval characters already mentioned which are common to both vertebrates and invertebrates, and, as we shall see later, there are many points which indicate that some of the best known living representatives of the Urochorda and Euchorda, such as the Ascidia and Amphioxus, have descended from animals which were more highly organized than their living representatives. This conclusion is partly based on general considerations and partly on the differences to be observed in the degree of development of the sense-organs in the larval state as compared with the adult animal; also the degenerate condition or absence of the same organs in the adult protochordate animal, as compared with those adult vertebrates in which the lateral eyes are certainly functional and also those, such as the cyclostomes and certain reptiles, in which the median eyes show a high degree of structural differentiation, which warrants the assumption that these median eyes also were functional in the ancestral stock from which the cyclostomes and reptiles have descended.
Fig. 123. — Balanoglossus. (After Spengel, from Parker and Haswell's
Textbook of Zoology.) br. : branchial regions. co. : collar. gen. : genital ridges.
hep. : projections caused by a series of paired hepatic pouches. pr. : proboscis.
Fig. 124. — The Apical Plate and Eyes of a Full-grown Tornaria Larva of Balanoglossus. (After Morgan.)
A. Apical view showing the relation of the plate to the longitudinal ciliated
band.
B. Antero-posterior, median section through the apical plate and eyes.
/. cil. b. : longitudinal ciliated band.
m.c : muscle cells belonging to the apical cord.
oc. a. : anterior eye.
oc. p. : posterior eye.
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Cite this page: Hill, M.A. (2024, April 19) Embryology Book - The Pineal Organ (1940) 13. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Pineal_Organ_(1940)_13
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
