282 MOLLUSCA. 
The cuttle is dicecious. 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 xidamental glands which secrete a sticky mass for 
fixing the eggs. The Zes//s 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 
is much yolk and the development is em- 
bryonic, with no larva. 
Reproductive, 
Development. 
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 plano-symmetry. Typically 
tridermic or triploblastic, the majority have a persistent 
ceelom, though there may be traced the same general 
tendency to a reduction of the perivisceral motor part, and a 
reciprocal expansion of the heemoccele 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 ccelom 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. 
