282 MOLLUSC A, 



The cuttle is dioecious The ovary is enveloped in an 

 ovisac and lies at the extreme hind dorsal end of the body. 

 The single oviduct leads to the exterior on 

 the left side of the mantle-cavity. There are 

 paired nidamental glands which secrete a sticky mass for 

 fixing the eggs. The testis lies in a similar position to the 

 ovary and is enclosed in a testicular sac continuous with 

 a vas deferens which swells into a seminal vesicle^ receives the 

 ducts of two prostate glands and opens along a penis into 

 the mantle-cavity. 



The eggs are laid on weeds in masses. They are black 



^ , ^ and like small grapes in appearance. There 



Development. . , n / ^u ^ i 



is much yolk and the development is em- 

 bryonic, with no larva. 



PHYLUM MOLLUSCA. 



The Mollusca are the second great division of the Metazoa. 

 Their external body-form may be very diverse but they 

 always have a fundamental piano- symmetry. Typically 

 tridermic or triploblastic, the majority have a persistent 

 coelom, though there may be traced the same general 

 tendency to a reduction of the perivisceral motor part, and a 

 reciprocal expansion of the haemocoele or venous-spaces. A 

 portion, however, remains as the pericardium, and it typically 

 communicates with the exterior by two specialised nephridia. 

 The gonadial part of the coelom in some cases still com- 

 municates with the pericardial. There is no trace of the 

 metameric segmentation which is so marked a feature of the 

 Annulata, though traces of archimeric segmentation persist. 



The nervous system consists of dorsal brain, a nerve-ring 

 and at least two other pairs of ganglia below^ the alimentary 

 "canal. Compound eyes are never found, but the simple eye 

 sometimes reaches a high state of perfection. The blood- 

 vascular system is usually well developed, the arteries being 

 nearly always definite vessels. The heart is typically three- 

 chambered, a median ventricle and paired lateral auricles, 

 and is always dorsal and systemic. 



The body itself is always soft and has no exoskeleton, 

 but there is usually a dorsal expansion called the mantle 

 which secretes a three-layered shell, either single or double. 



