[110] The probability of the choroid gland having the meaning attributed to it by Dohrn is strengthened by the existence of a præmandibular segment as evidenced by the presence of a præmandibular head-cavity, the walls of which as shewn by Marshall and myself give rise to the majority of the eye-muscles and of a nerve (the third nerve, cf. Marshall) corresponding to it; so that these parts together with the choroid gland may be rudiments belonging to the same segment. On the other hand the absence of the choroid gland in Ganoidei and Elasmobranchii, where a mandibular pseudobranch is present, coupled with the absence of a mandibular pseudobranch in Teleostei where alone a choroid gland is present, renders the above view about the choroid gland somewhat doubtful. A thorough investigation of the ontogeny of the choroid gland might throw further light on this interesting question, but I think it not impossible that the choroid gland may be nothing else but the modified mandibular pseudobranch, a view which fits in very well with the relations of the vessels of the Elasmobranch mandibular pseudobranch to the choroid. For the relations and structure of the choroid gland vide F. Müller, Vergl. Anat. Myxinoiden, Part III. p. 82.
It is possible that the fourth nerve and the superior oblique muscle of the eye which it supplies may be the last remaining remnants of a second præmandibular segment originally situated between the segment of the third nerve and that of the fifth nerve (mandibular segment).
[111] I do not mean to exclude the possibility of the mandibular arch having supported a suctorial mouth before it became converted into a pair of jaws.
[112] Dohrn (No. [250], p. 25) gives an explanation of the origin of the new anus which does not appear to me quite satisfactory.
[113] As pointed out in Vol. II. p. 255, there is a striking similarity between the history of the neurenteric canal in Vertebrates, and the history of the blastopore and ventral groove as described by Kowalevsky in the larva of Chiton. Mr A. Sedgwick has pointed out to me that the ciliated ventral groove in Protoneomenia, which contains the anus, is probably the homologue of the groove found in the larva of Chiton, and not, as usually supposed, simply the foot. Were this groove to be converted into a canal, on the sides of which were placed the nervous cords, there would be formed a precisely similar neurenteric canal to that in Vertebrata, though I do not mean to suggest that there is any homology between the two (vide Hubrecht, Zool. Anzeiger, 1880, p. 589).
[114] Vide the chapter on the Germinal Layers.
[115] In the Chætopods various organs have been interpreted as rudiments of a notochord, but none of these interpretations will bear examination.
[116] “Ueb. d. Vorkommen eines schwimmblasenähnlichen Organs bei Anneliden.” Mittheil. a. d. zool. Station zu Neapel, Vol. II. 1881.
[TN1] Transcriber’s Note. The following are included in the illustration: Mammalia, Sauropsida, PROTO-AMNIOTA, Amphibia, Teleostei, PROTO-PENTADACTYLOIDE, Ganoidei, Dipnoi, PROTO-GANOIDEI, Holocephali, Elasmobranchii, PROTO-GNATHOSTOMATA, Cyclostomata, PROTO-VERTEBRATA, Cephalochorda, PROTOCHORDATA, Urochorda.
[117] The openings of the hepatic diverticula through the sacks lined with thread cells are described by Hancock and Embleton, Ann. and Mag. of Nat. History, Vol. XV. 1845, p. 82. Von Jhering has also recently described these openings (Zool. Anzeiger, No. [23]) and apparently attributes their discovery to himself.