1904.] NATURAL SCIENCES OF PHILADELPHIA. 259 



would be continually obscured and would be practically useless even 

 if provided with special tentacles. It seems most reasonable to sup- 

 pose that as the structures characteristic of the chitons appeared in 

 the phylogenetic development, the eye-spots gradually shifted their 

 position into the present more favorable location. 



Pelseneer' has made a detailed study of the larval eyes of some of 

 the Mytilidcc and the related genus Avicula. They arise in the embryo 

 behind the velum and on the base of the first gill-filament. "Each 

 eye is open, that is to say, an invagination of the skin, and consists of 



pigmented epithelial cells The cavity is filled by an elongated 



crystalline body continuous with the overlying cuticle They 



have a structure intermediate between the eyes of Patella and Trochus," 

 and are innervated by fibers from the cerebral ganglia. Pelseneer con- 

 siders that this type of eye and that of the chitons are homologous. 

 This assumption must rest entirely upon the fact that both are post- 

 trochal. They certainly are fundamentally different in structure. 

 Even with Chiton polii this is the case, and, furthermore, this organ in 

 the lamellibranchs is innervated by nerves from the cerebral ganglia, 

 and in the chitons by the pallial nerves. Thiele's* contention that the 

 eye of Area noce and the chiton eye are homologous rests upon the same 

 foundation as Pelseneer's argument. Both are behind the velum, 

 but fundamentally different structurally and in their innervation. As 

 the matter now stands, the theory that the chiton and the annelid eye 

 are homologous rests upon identity of structure ; while the chiton and 

 lamellibranch larval eye are supposed to be genetically related 

 because of similarity of position. 



It is improbable that the eyes of chitons are functional only in the 

 larvae. In the three species studied before the metamorphosis the pig- 

 ment appears about twenty-four hours prior to the free-swimming 

 stage, which lasts from fifteen minutes to twenty-four hours, according 

 to conditions. After the metamorphosis, which ensues after the 

 embryos have settled, these sense organs in at least eight species 

 invariably persist for a considerable length of time. In fact, they 

 appear to remain as long as the shell and mantle are sufficiently trans- 

 parent to allow the light to penetrate, or until the animal is upward of 

 5 mm. in length. Some of the smaller species are at this time sexually 

 mature; while some of the larger forms are only one-fourth or even 

 one-tenth their adult size. 



^ "Les yeux cephaliques cliez les Lamellibranches," Arch, de Biol., T. 16. 

 * "Ueber Sinnesorgane der Seitenlinie und das Nervensystem von Mollusken," 

 Zeit. f. IV. Zool, Bd. XLIX. 



