260 



GENERAL CONSIDERATIONS. 



species a typical radula is present together with dorsal and ventral salivary 

 glands and a diffuse digestive gland. In the posterior end of the body great 

 coenogenetic changes have ensued. The coelomoducts are usually united, semi- 

 nal receptacles have appeared, and a highly glandular epithelium has developed 

 at least in the terminal section. Penial spines and branchial folds are, in my 

 opinion, likewise -recent developments. Where the spines imbedded in the 

 cuticle investing the body develop from a single matrix cell it probably represents 

 the most primitive condition, but since certain neomenians develop some of 

 their spicules in the same general fashion as the Chitons do in part it appears 

 probable that both modes prevailed in the ancestral Solenogastre. 



As noted above the Solenogastres are a highly variable group, and such 

 genera as Alexandromenia, Pachymenia, and Neomenia appear to me to stand 

 among the most highly modified members of the order Aplacophora. The 

 Chitons on the other hand are a remarkably conservative group, the differences 

 between the most diversified genera, such as Cryptochiton, Chitonellus, and 

 Ischnochiton, being far less than those differentiating Limifossor and Chaeto- 

 derma belonging to the same family. When I made the claim that the Chitons 

 represent the most archaic type of modern mollusc I had in mind the highly 

 modified Solenogastres just noted which appear to me to have departed more 

 widely from the ancestral mollusc than any of the Chitons. It doubtless is 

 possible to select a character here and there from the various species of known 

 Solenogastres and produce a list of primitive features of greater length and more 

 importance than in the case of the Chitons. On such a basis of selection the 

 Solenogastres may be considered to be the more primitive group, but where 

 a single species of Solenogastre (especially from one of the genera noted above) 

 is compared with a single species of Chiton it appears to me that more primitive 

 features will be found to exist in the last named. However this is not a matter 

 upon which I would lay great stress since it appears to rest upon much less con- 

 clusive evidence than does the theory whereby the Solenogastres are considered 

 to be more closely related to the Chitons than to any other group of molluscs. 



