GENERAL CONSIDERATIONS. 167 



In the Neomeniina there is the same cleft in the ventral musculature, 

 thickened as in the Chaetodermatina, and the foot is present as a small fold 

 extending along the mid ventral line. Anteriorly it affords an outlet for the 

 enormously developed anterior pedal gland which in position and development 

 as far as this has been traced (see Heath '05), is homologous with the pedal 

 gland of young Chitons. The remainder of the foot is supplied with the poste- 

 rior pedal glands which are present in a diffuse condition in the Chiton foot. 



In the Solenogastres there are no eyes, tentacles and even the proboscis or 

 snout of the Chitons is believed generally to have no homologue in the group. 

 Concerning this last named organ however Thiele claims (and in this he is fol- 

 lowed by Nierstrasz and Pelsenecr) that it is present, though in a highly modified 

 condition and I am strongly convinced of the force of his argument. In the 

 first place it is a well-known fact that the first section of the alimentary canal 

 in the Neomeniina contains the atrial ridges (Mundleisten) and the enclosed 

 cirrose area, all innervated by fibres originated directly by the cerebral ganglia. 

 In Rhopalomenia aglaophetiiae, Dinomenia hubrechti, etc., and I have found the 

 same state of affairs in Driomenia pacifica, this portion of the canal exists in the 

 form of a depression in front of the mouth, and is separated from it by a narrow 

 bridge of spicule bearing cells continuous with the general covering of the body. 

 Owing chiefly to its innervation Thiele considers that this "sensibles Atrium" is 

 the homologue of the Chiton snout which is now withdrawn into a depression. 

 Where this atrium is a direct part of the digestive tube the true mouth has been 

 drawn into the body and is located immediately behind the most posterior atrial 

 ridge. It now becomes an interesting fact that in the Chiton development the 

 mouth at first is posterior to the snout and but slowly takes up its final central 

 position (cf. Heath '99). It lends sujiport to the belief that in these molluscs 

 with isolated atrial cavity the position of these organs does not represent a 

 highly modified condition but a relatively primitive state of affairs. 



In the Chaetodermatidae the nerves which originate from the anterior 

 surface of the brain pass at once into the huge, compact ganglionic bodies homol- 

 ogous, I believe, with the more diffuse nerve masses in contact with the bases 

 of the cirri and buccal sensory ridges in the Neomeniidae. From these a rela- 

 tively enormous number of fibres pass into the Mundschild or buccal plate, 

 just as the cirri and ridges are innervated by nerves from the neighboring ganglia. 

 There is thus little doubt that the buccal plate and sensory atrium are homolo- 

 gous and if the above line of reasoning be correct, they are the homologue also 

 of the Chiton snout, which is likewise innervated by nerves from the cerebral 

 ganglia. 



