GASTROPODA ANISOPLEURA. 481 



The vagina is always short, and there is in some instances an appended 

 receptaculum seminis, e.g. Paludina, Natantia, The genital opening is 

 usually at some distance from the anus. In hermaphrodite forms there is 

 an hermaphrodite duct to the ovo-testis, which may remain outwardly 

 undivided as far as the aperture, but has a partially divided cavity, as in 

 Aplysia and most other Opisthobranchia Palliata. It usually divides, 

 sooner or later, into separate male and female ducts. A ciliated furrow 

 leads from the genital aperture to a hollow evaginable penis on the right 

 side of the head in Aplysia and a few others. When the ducts are separate, 

 the vas deferens ends in an evaginable penis, and has commonly an 

 appended prostatic (?) gland. A vesicula seminalis is often present. The 

 oviduct has a uterine section, an attached receptaculum seminis, and 

 invariably an albuminiparous gland. The two apertures, male and female, 

 open as a rule, except in aquatic Ptdmonata, into a sexual cloaca, which 

 lies either near the anus, or on the right side of the body, and in all 

 Pulmonata far forwards, close to the head. Some Pnlmonata and certain 

 species of Doris possess a dart, attached in the former to the female, in the 

 latter to the male, duct. 



Two forms of spermatozoa are found in the same individual in various 

 Azygobranchia, and of these one form only appears to fertilise the ovum in 

 Paludina. In the hermaphrodite groups copulation may or may not be 

 reciprocal. The ovum is small, and is usually devoid of a vitelline mem- 

 brane. It is surrounded by a quantity of albumen, the surface of which 

 hardens. In some of the Pulmonata there is a calcareous egg-shell, e.g. 

 Testacella, Helix. In some Azygobranchia, e. g. Paludina, and some Pul- 

 monata (e. g. Clausilia, Pupa), the ova develope within the uterus, but in 

 most cases they are laid, either singly or in masses, contained sometimes in 

 a fixed hard cocoon with terminal aperture, as in many Azygobranchia. 

 One ovum only frequently developes in this case, the others serving as food 

 material. The egg-masses are usually fixed to some object, except in the 

 terrestrial Pulmonata. They are carried about attached to a float-like 

 structure formed by the foot in lanthina. Segmentation is unequal, but 

 the degree of inequality depends upon the amount of food-yolk present. 

 The hypoblast cells are sometimes of very large size, and two yolk cells 

 persist for a long time in Aplysia. When the blastomeres are all relatively 

 small, the gastrula is invaginate, e.g. Paludina ; when the hypoblast cells 

 are large, it is formed by overgrowth. There is a Trochosphere and Veliger 

 stage. The velum is said to be absent in some Pidmonata. It usually 

 becomes bilobed or multilobed, and is large in size in Natantia, and 

 persists to a greater or less extent in the Opisthobranch Tergipes and the 

 Pulmonate Limnaeus. 



Most Gastropoda Anisopleura are marine ; the basommatophorous 

 Pulmonata, and some Azygobranchia, e. g. Paludina, Neritina, are fresh- 



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