354 THE PINEAL ORGAN 



resembling the anterior chamber of a vertebrate eye, but differing from 

 it in its mode of formation and in communicating through an unclosed 

 opening with the exterior and thus being filled with sea-water. We have 

 thus, by means of what is termed " parallel development," an eye which 

 resembles in many respects a vertebrate eye, yet differs from the verte- 

 brate type in some very important structural details, which indicate 

 that the differentiation and perfection of the cephalopod type of eye, 

 must have taken many ages to complete and that the divergence of the 

 parent stem of these higher molluscs must have taken place at a very 

 early period in the phylogeny of the invertebrate stock. This conclusion 

 is supported by reference to the ontogenetic development of Sepia, 

 which was worked out by Koelliker and also by geological evidence. 

 Embryology shows that there is a marked deviation in the course of 

 development of the higher Mollusca from that of the simpler types, such 

 as the Amphineura, which include Chiton (Fig. 115, Chap. 12, p. 157) and 

 the Aplacophora, in which the bilateral symmetry of the central nervous 

 system — which consists of paired lateral and ventral cords provided with 

 serially arranged ganglia and united by transverse commissural bands, 

 as in Turbellaria — remains undisturbed ; and, further, that the remark- 

 able specialization in form which characterizes each of the higher classes 

 of the Mollusca has not been evolved. While on the geological side, 

 judging from the similarity of the phragmacone of living cephalopods to 

 the ammonites, and belemnites of the Lias strata which belong to the 

 lower series of the Jurassic period, it may be inferred that the Cephalopoda 

 of that time were not only highly differentiated as regards their soft 

 parts, including the eyes, but much larger than any living species. It 

 seems obvious also that the period during which this remarkable differ- 

 entiation took place in the higher types of the Cephalopoda must have 

 extended much farther back than the Lias, in which it seems to have 

 already attained a maximum. 



If one were to judge from the higher types of Mollusca only, the 

 differences which exist in the eyes and general anatomy of these types 

 might lead one to think that the whole phylum of Mollusca was totally 

 different from the phyla of other invertebrates. This is not so, however, as 

 is most clearly seen in the ontogeny and in the adult structure of the 

 Amphineura. The development of this class was specially studied by 

 Kowalevsky in Chiton. In this animal the early stages of development 

 show a typical blastocyst, gastrula, mouth (blastopore), proctodeum and 

 anus, mesoderm formation, paired eyes and otocysts, cerebral ganglia, 

 lateral (pleuro-visceral) and ventral (pedal) nerve cords — all of which 

 point to a primary community of origin of the Mollusca, with other 

 phyla of the invertebrates, and also indicate, when compared with similar 



