MOLLUSCA. 119 



branched structures, surrounding the coils of the intestine in 

 the visceral mass (Fig. 32, G). They are yellowish in the male, 

 reddish in the female, and consist of an immense number of 

 blind tubules, from which ducts pass on either side, uniting 

 together into a short spermiduct or oviduct which opens to the 

 exterior by the genital aperture (Fig. 33, Oe'). 



The tubules of the gonads are lined by germinal epithelium. 

 These in the male produce large numbers of sperms, each of 

 which possesses a cylindrical head and motile filiform tail. In 

 the female ova are developed, each of which is covered by a 

 vitelline membrane, which at one point, the micropyle, is incom- 

 plete. The vitellus, which contains a moderate amount of food- 

 yolk, is separated when ripe by albuminous fluid from the 

 vitelline membrane. It contains a large germinal vesicle with a 

 germinal spot. Each ovum while still in the ovary is contained 

 in a follicle, to the wall of which it is attached by a protoplasmic 

 stalk passing through the micropyle. 



The ova pass back from the genital openings to the cloacal 

 chamber, and then forwards into the water-tubes of the outer 

 gill-plates, where they are fertilized by sperms brought in by the 

 inhalent current. The sperm enters the ovum by the micropyle. 

 The breeding season is in the summer. 



8. Muscular System. The skin is intimately connected withx 

 underlying muscle, and there are also definite muscles, of which \ 

 the adductors have already been described. The foot is almost / 

 entirely made up of muscular tissue, and slender protractor, anterior I 

 retractor, and posterior retractor muscles, which take origin from I 

 the corresponding impressions on the shell are inserted into it. 



The muscles are composed of fusiform muscle-cells, exhibiting 

 somewhat indefinite transverse striations. The foot serves as an \ 

 organ of locomotion by which the animal can move forwards in \ 

 the mud, leaving a furrow-like trail behind it. The retractor J 

 muscles by their contraction draw the foot back, and the pro/ 

 tractors pull it forwards. 



9. The nervous system (Figs. 32, 33, and 34) consists of a wide 

 oesophageal nerve-ring, a long visceral loop connected with this 

 on the dorsal side, and numerous nerves. Two pairs of ganglia 

 (cerebro-pleural and pedal) are developed upon the ring, and one 

 pair (visceral) upon the loop. It is convenient to use the term 

 commissure for a nerve-cord connecting ganglia of the same name, 



