194 BULLETIN 0¥ THE 



Mv. Dall found the sexes always separated in all the species 

 examined (belonging to thirty or forty generic or subgeneric 

 groups), including Chiton Fallasii, which Middendorf had sup- 

 posed to be hermaphrodite. The egg is impregnated in the ovi- 

 sac, or oviduct, or both, in several species ; whether in all or not 

 further material was needed to determhie; in none of the species 

 examined was a chorion discovered ; nevertheless it may exist in 

 some and not in others. Mr. Dall detected the renal organ in 

 many species, but it was very small, or even perhaps abortive in 

 some species ; he did not discover the excretory external opening,, 

 but had not sought especially for it. An oviduct (or pair of ovi- 

 ducts) exists demonstrably in some species, having a small plain 

 opening on each side of the tail of the animal ; in others no ovi- 

 duct could be discovered, and the small opening was replaced by 

 a larger fenestrated opening, apparently giving free access to the 

 water into the perivisceral cavity. From these openings the eggs 

 had been seen to be ejected by Dr. Carpenter and Prof. Verrall, 

 and before ejection had developed to such an extent that the em- 

 bryonic eyes were plainly visible. (See Bulletin Essex Inst. 1873.) 



Mr. Dall saw nothing of striated muscular fibre among the 

 C'hitonidse dissected. In none of them was a chitinous jaw found, 

 such as is universal among limpets. The dentition, while differing 

 in detail in different species, was of similar type in all examined, 

 and probably in all chitons. A laminated crop exists in most- 

 species. One peculiarity is notable throughout the group. The 

 tendency is to degradation of cephalic characters. In embryo 

 chitons the eyes are well developed and the cephalic portion 

 largely developed. In adult individuals not only are there no 

 eyes, but the tentacles common to most gasteropods are absent, 

 the nerves which are wont to supply these organs with sensibility 

 are also wanting ; there is no jaw, the important centres of cir- 

 culation, respiration, and reproduction are all posteriorly situated. 

 As between different genera of chitons, those which have the 

 rows of branchial tufts shorter than in other genera have the 

 deficiency in the cephalic end of the row; in Chitoncllus, per- 

 haps the highest form of Ghitonidse, the branchiae are few, large, 

 and collected in a very short bunched-up row close to and on each 

 side of the anus. 



The chitons go back to Silurian times and (excluding the cer- 

 riped valves, fish-scales, etc., which have been described by Eu- 



