xn 



PHYLUM MOLLUSCA 



725 



and 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 

 the pericardium a specialised part of the ccelome, communicating 

 with the cavity of the kidneys. 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 Strep- 

 toneura at the side or behind. Given off from the apex of the 

 ventricle is a large vessel which soon bifurcates to form anterior 

 and posterior aortae. 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 arrangement of the 

 ganglia and their commis- 

 sures and connectives. 



In the majority the 

 arrangement 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. 645) widely FIG. 645. 

 separated, situated over the 

 gullet, and giving off behind 

 a pair of 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 a paired ganglion, as in Triton). 

 In the course of these visceral cords there is also a pair of visceral 

 ganglia. A pair of pedal ganglia, united together by a transverse 

 commissure and joined to the cerebral and pleural ganglia by 

 connectives, give off behind one or two pairs of pedal nerves, as 

 already mentioned. A pair of buccal ganglia are connected by 

 slender nerves with the cerebral. At the base of each osphradium 

 is usually a small osphradial ganglion connected by a slender nerve 

 with the visceral. In most Streptoneura (Fig. 629), in accordance 

 with the torsion of the visceral mass, the visceral cords are twisted, 

 as already described in the case of Triton, into a figure of 8 : the 

 right visceral ganglion becomes displaced to the left and comes 

 to lie above the alimentary canal (supra-intestinal), while the 



Nervous system of Patella, cer. g. 

 cerebral ganglia ; mant. n. mantle-nerves ; 

 osphr. g. osphradial ganglia ; ped. g. pedal 

 ganglia and pedal nerve-cords ; pi. g. pleural 

 ganglion. (After Spengel.) 



