124 SEXUAL ORGANS. 



consisting of two-thirds of a circle, narrowed into a neck at top, 

 is produced by the American Naticas, N. heros and N. duplicata 

 (xvii, 95). It is built up of eggs cemented together by sand 

 and a gelatinous material ; and is very common on our sea- 

 beaches ; the egg-pouches of Nassa are frequently attached to 

 its inner wall. 



Janthina attaches its eggs to the vmder side of a float (xvii, 

 99) composed of numerous cartilaginous air-vesicles, and secreted 

 by its foot. Lamellaria perspicua digs a cavity in the colonies 

 of certain compound Ascidians, upon which it usually feeds, and 

 there oviposits. 



Some Polynesian Helices (Endodonta) having a large umbil- 

 icus or basal axial opening in their shells, oviposit in it and 

 cover the aperture with a thin diaphragm to prevent them from 

 falling out. The eggs of Hipponyx, Capulus and Calyptrsea 

 are attached to the body of their mother, who thus appears to 

 protect them, like the Janthina. On the other hand, the 

 Limnseids not only take no care of their eggs, but are knoAvn to 

 devour them. 



Scaphojjoda. The sexual organs, like some other portions of 

 their anatomy, are more nearly related to the bivalve than to the 

 encephalous mollusca. The sexes are separated, and the organs 

 are composed of two symmetrical glands, the products of which 

 are excluded through a posterior orifice, and through the open 

 posterior end of the shell. 



Acephala or Pelecyjooda (xviii, 11, 17 ; xxii, 64). The sexual 

 organs are composed of genital glands and vector channels. The 

 genitals are S3^mmetrical, paired and placed at the sides and base 

 of the visceral mass ; they are sometimes prolonged to the foot, 

 sometimes to the lobes of the mantle. " Pecten viaximua, P. 

 Jacobseua, P. glaher^ Gardium, Pennaiiti^ Pandora insequivalvis^ 

 Anodonta cellenais and Ostrea edulis have been recorded as her- 

 maphrodite, and Pecten varius^ Gardimn edule, and most species 

 of IJnio as dioecious. But inasmuch as the male and female glands 

 are not developed at the same time, it follows that an individual 

 examined at different epochs may appear to be male or female 

 or hermaphrodite. It is probable that some of the above deter- 

 minations are therefore erroneous, and that more extended 

 observation will greatly increase the number of dioecious bivalve 

 mollusks. The difficulty in determining sex in these animals is 

 increased by the fact that a single excretory canal serves for both 

 ovules and spermatozoa. The male gland is whitish, the ovary 

 usually reddish (Spondylus, Janira, Mytilus) ; these two glands 

 are lobulated and generally distinct enough, but in the European 

 Ostrea the spermatic and ovarian follicles are contiguous and, 

 so to say, confounded." 



" In some species (European Ostrea) the disposition of the 



