NATURE OF THE INVERTEBRATE BRAIN. 31 



the cockle, Venus, and other bivalves possessing those prolongations 

 of the mantle known as siphon-tubes, the eyes are situated either at 

 the base or on the tips of the numerous small tentacles distributed 

 around the orifices of these tubes, which in those of them living in 

 the sand are often the only parts appearing above the surface. The 

 margins of the mantle are also garnished by a number of short though 

 apparently very sensitive tentacles, in which the creature's most spe- 

 cialized sense of touch seems to reside. Some of these tactile append- 

 ages, as well as some of the ocelli, send their nerves to the posterior 

 or parieto-splanchnic ganglia, while those situated on the anterior 

 borders of the mantle communicate with the anterior or oral ganglia. 

 The latter ganglia also receive filaments from the so-called labial ap- 

 pendages, whose function is uncertain, though it has been suggested 

 that they may be organs of taste or smell. Lastly, in close relation 

 with the pedal ganglia or ganglion, there are two minute saccules 

 (Fig. 9, s), to which an auditory function is usually ascribed. 



Thus we find among these headless mollusks a distribution of spe- 

 cially impressible parts or sensory organs, such as cannot be paralleled 

 among any other animals. The sense of touch and the sense of sight 

 seem to be more especially in relation with the great posterior gan- 

 glia. These sensory functions are, however, to a minor extent shared 

 by the oral ganglia, which are also in relation with parts that may 

 possibly be organs of taste or smell. On the other hand, auditory 

 impressions are invariably brought into relation with the inferior or 

 pedal ganglia. In these headless mollusks, therefore, the functions 

 pertaining to the brain in other animals are distributed in a very re- 

 markable manner, and the anterior ganglia cannot in them be proper- 

 ly regarded as representing such an organ. 



The viscera in these lamellibranchs are also in relation with the 

 three pairs of ganglia, and not exclusively with any one of them. 

 Filaments to the intestinal canal and the liver are usually given off 

 from the commissures between the anterior and the posterior ganglia; 

 the genital organs are in connection with filaments coming from the 

 commissures between the anterior and the inferior or pedal ganglia ; 

 while the branchiae are in relation with the ganglia at the posterior 

 part of the body. 1 



There is another interesting class of mollusks the Pteropoda 

 which,, in respect of powers of locomotion and the possession of a dis- 

 tinct head, may, if for no other reasons, be said to lead us on from the 

 comparatively sluggish bivalve Mullmca to the gasteropods and the 

 cephalopods, all of which are distinguished by definite and wide- 

 reaching powers of locomotion, and by the possession of a distinct 

 head carrying sense-organs, and a more or less developed brain. 



1 In speaking of the nervous system of the lamellibranchs, I have not alluded to cer- 

 tain small accessory ganglia which exist in some of them in relation with peculiar special- 

 ly developed contractile structures. 



