302 Introduction to Animal Morphology. 



viviparous. Fixed forms, like Vermetus, are impregnated by 

 the diffusion of spermatozoa in the surrounding water. Chi- 

 tons alone have symmetrical ducts, and a median ovary lying 

 on the aorta. They have rarely any appendages except a 

 penis, often placed on the right side of the head below the 

 eye. The ovary is embedded in the liver, and has a tortuous 

 oviduct. Neritina and Paludina have accessory albumen 

 glands, and its uterine wall secretes myelin. There may be 

 a prostate in the male (Natica), often a vesicula seminalis. 

 Tin- i>mis is absent in Trochidoe and Scutibranchs ; in others 

 it is present, and has a claw-like appendix in Dolium and a 

 penial gland in several other genera. 



The eggs are laid usually in chains or in gelatinous 



masses, or contained in nidamental capsules. The masses 



may be (<j) irregular, with the eapsules fixed one to the other 



; < -olKerentes), dehiscing by a slit or by a round operctilige- 



rous opening; or with the capsules in a common membrane 



attached to foreign bodies (adhctrentes). These may dehisce 



lit, or by a round opening, and the capsules may be 



e,* or stalked, cyathiform, oviform, or funnel-shaped ; 



otherwise, they may be in (//) regular masses, coherent or ad- 



:, with a central axis of attachment, as in Pyrula, on 



which they may be sessile or stalked. 



Tii.- ire sometimes large, with calcareous shells 



(Puhnonata) ; the yelk undergoes total cleavage, and the body 

 is built of the peripheric cells of the blastoderm. The larva 

 develops a velum and ciliary circlet, and beneath the velum 

 forms the mouth, intestine, and anus. The foot begins as a 

 ciliary process below the mouth, and then the mouth and 

 shell form, then the ear cysts appear, afterwards the tentacles 

 and eyes ; the anus, at first terminal, becomes anterolateral, 

 and the embryo becomes free-swimming. f The shells of the 



* Purpura capsules contain from 30 to 150 eggs, but only one usually 

 reaches perfection. 



f In a Prosobranch larva there are four stages : 1st. From the begin- 

 ning of rotatory movement to the formation of a general ciliary clothing. 

 2nd. To the formation of a velum, foot, mantle, and shell. 3rd. The de- 

 velopment of the first spire of the shell, of the heart, and the primary 



