XII 



PHYLUM MOLLUSCA 



In some cases, e.g., Haliotis, it traverses the ventricle, in others 

 the pericardium ; in others it passes through the nephridium. In 

 Eolis (Nudibranchia) the stomach gives off a number of glandular 

 coeca which penetrate into the interior of the secondary branchiae 

 or cerata on the dorsal surface ; these coeca take the place of the 

 liver of other Gastropoda. In some of the Pectinibranchia there 

 is a peculiar adrectal gland situated at the side of the rectum 

 secreting a colourless fluid which in Murex and Purpura turns 

 purple on exposure to the air, and was anciently used as a dye 

 the " Tyrian purple." 



The heart is as in other molluscs enclosed in a special cavity- 

 trie pericardium a specialised part of the ccelome, communicating 

 with the cavity of the nephridia. It consists usually, as in 

 Triton, of two chambers auricle and ventricle but in some, 

 e.g., Haliotis, there are two auricles and a ventricle. In the 

 Opisthobranchia, as already mentioned, it lies in front of the 

 ctenidia ; in the Streptoneura at the side or behind. Given off 

 from the apex of the ventricle is a large vessel v/hich soon bifur- 

 cates to form anterior and posterior aortre. These are the main 

 trunks of the arterial system, which is more highly developed than 

 in the Pelecypoda ; the finest branches terminate in sinuses as in 

 the latter class. 



The nervous system varies considerably in the different 

 groups in regard to the ar- 

 rangement of the ganglia 

 and their commissures. 



In the majority the ar- 

 rangement is nearly that 

 which has been described as 

 occurring in Triton. There 

 is a pair of cerebral ganglia 

 usually closely united, but in 

 Patella (Fig. 597) widely 

 separated, placed over the 

 gullet, and giving off behind 

 a pair of nerve-cords the 

 visceral nerve-cords in the 

 course of which there is placed 

 laterally a pair of pleural 

 ganglia, and which are united 

 together behind in a median 

 abdominal ganglion (or two 

 as in Triton). In the course 



of these visceral cords there is a pair of visceral ganglia. A 

 pair of pedal ganglia united together by a transverse com- 

 missure and joined to the cerebral ganglia by connectives, 

 give off behind one or two pairs of pedal nerves as already 



Y Y 2* 



FIG. 597. Nervous system of Patella, cer. \i. 

 cerebral ganglia ; nni,it. n., nint. ,>. mantle 

 nerves ; osp/i. g. osphradial ganglia ; pad. <i. 

 pedal ganglia ; pec/, n. pedal nerve cords ; 

 t'l.(i. pleural ganglion. (After Spengel.) 



