282 INVBRTEBBATE MORPBOLOQT. 



with either the cerebro-pedal or pleuro-visceral system may 

 be developed, the most constant of which are the huccal gan- 

 glia (6m) which lie at the sides of or more usually below the 

 buccal mass which they innervate and are united by commis- 

 sures with the cerebral ganglia. Two nerve-rings in such 

 cases surround the oesophagus, i.e., that formed by the cere- 

 bro-pedal and that of the cerebro-buccal connectives. 



This description has reference only to what may be con- 

 sidered a typical condition, and it must be remembered that 

 frequent modifications of it may occur. In the Gasteropods, 

 for example, in which, in accordance with the development of 

 a circular mantle-fold, the anus comes to lie on the anterior 

 portion of the body-wall, a peculiar crossing of the pleuro- 

 visceral commissures occurs in some cases, and as a result 

 what was originally the right parietal ganglion comes to lie 

 upon the left side of the body and the original left ganglion 

 upon the right side. Further consideration of this arrange- 

 ment may, however, be postponed until the Gasteropods are 

 under discussion. Mention should, however, be made here 

 of another not unfrequent modification of the typical arrange- 

 ment of the nervous system, which consists in the concentra- 

 tion of the ganglia and the shortening of the various connec- 

 tives. This may affect only the cerebral, pedal, and pleural 

 ganglia, bringing them into close approximation, or, as in some 

 Cephalopods, the visceral ganglia may also be carried forward 

 so that all the principal ganglia are united into a single lobed 

 mass closely surrounding the oesophagus behind the pharynx. 

 This condition constitutes of course the culmination of the 

 concentration process, but various gradatious of it are to be 

 found in the different groups. 



Sense-organs are as a rule well developed in the Mollusca 

 and descriptions of many of them may be more conveniently 

 given in connection with the detailed account of the various 

 groups. The general ectoderm of the mantle and body- wall 

 has scattered in it numerous sensory cells which may become 

 specially aggregated at certain points to form definite sense- 

 organs. Thus tentacles are frequently borne upon the head 

 which are tactile or in some cases olfactory in nature, and at 

 the bases of the gills special aggregations of sensory cells are 



