THE NEETOUS SYSTEM OP THE MOLLTJSCA. " 181 



before (i. e. cut out, exposed to the air, and cut into sections) 

 showed sensory cells most satisfactorily. The sensory cells in the 

 osphradial epithelium could also be thus differentiated ; but a 

 special difficulty was met with in the tentacles, owing to their 

 immediate and prolonged contraction on the slightest irritation, 

 by which the reagent was driven completely out of them. 

 Attempts were made to force the blue into the tentacles by 

 various devices ; but the difficulty was only finally got over by 

 injecting the animal with cocain (5 per cent, in sea-water) and 

 a few minutes afterwards injecting the blue, which then readily 

 penetrated the relaxed tentacles. Under this treatment, how- 

 ever, the animal {Aplysia) died much sooner than when the blue 

 was alone injected ; and it is well known that the colouring- 

 matter employed rapidly diffuses on the death of the tissues. 

 Only a few sensory cells were seen in sections thus prepared ; and 

 as no more animals were forthcoming the experiment was not 

 repeated. 



Material. 

 The animal with which the best results were obtained, and 

 which was easiest to manipulate on account of its size and 

 absence of a hard shell, was Aplysia. The nervous system of 

 Aplysia is, moreover, not so concentrated as that of the Nudi- 

 branchs, and the cells and fibres are fairly large in contrast to 

 those of the Prosobranchs. Moreover, the stain did not seem 

 suitable for other Opisthobranchs, as in the animal tried (Doris) 

 it was found to become, as it were, scattered, the fibres having 

 apparently broken up into segments. The attempt to stain 

 small transparent Nudibranchs by leaving them in a weak 

 solution was unsuccessful. With the Prosobranchs only a 

 partial success was obtained in the case of Patella. No success 

 at all was met with in the treatment of Laraellibranchs ; but 

 I have no doubt that the method could be applied to all Mollusca 

 if time and trouble were taken to determine the treatment 

 suitable to individual cases. 



Central Nervous System. 

 Pig. 1 diagrammatically delineates a section through the 

 nervous system of Limncea, and will give a general idt-a of the 

 relation of its parts, and the relative size and distribution of the 

 ganglionic cells and nerve-fibres. The arrangement is typically 

 that met with in the Pulmonata and Opisthobranchiata, the various 



13* 



