426 THE ANATOMY OF INVERTEBRATED ANIMALS. 



which may take the form of a special albumen gland, and a 

 spermatheca opens into it near its outer extremity; while, on 

 the male side, a vesicula seminalis and an eversible peuis 

 may be added. The penis, however, may be distant from the 

 genital opening, and then a groove on the side of the body 

 leads to it (Aplysia). In the latter case there are two geni- 

 tal apertures, one for the male and one for the female organs, 

 though they may open into a common vestibule. The penis 

 is an eversible involution of the integument, on which the vas 

 deferens opens. A prostate gland is usually connected with 

 the latter, and, near its opening, there may be a saccular ap- 

 pendage, in which a hard pointed body, the spiculum amoris, 

 is contained (Don's, Helicidce). An albumen-gland opens 

 into the uterus, and a spermatheca is connected with the 

 vagina. 



Spermatophores, by the aid of which the spermatozoa are 

 transferred into the female organs, occur in the Cephalopoda, 

 and in the Palmonata. In the latter they are grooved bands, 

 or incomplete tubes of hardened mucus secreted by the penis, 

 which become filled with spermatozoa during copulation ; 

 while, in the former, they are closed cases which may have a 

 very complex structure. 



In the great majority of the Oclontophora the young 

 leaves the egg as a vellger, very similar to that of the Lamel- 

 libranchiata. The velum usually becomes bilobed, and some- 

 times (Hetero})oda) its margins are produced into many ten- 

 taculiform processes ; and, in all Pteropoda and Pranchio- 

 (j'lsteropoda, whether the adult possess a mantle and a shell 

 or not, the larva is provided with both, the shell being at first 

 a simple conical symmetrical cap, developed in the middle line 

 of the mantle. The eyes make their appearance behind the 

 velum, and the tentacles in front of or upon it. 



While the course of the development of the embryo in 

 the O lontophora presents a general uniformity, there are 

 wide differences in detail. 



In Paludina* the blastomeres produced by yelk-division 

 are of equal size. They arrange themselves into a vesicular 

 morula, which undergoes invagination and becomes a gas- 

 trula of the simplest tyjie. The aperture of invagination 

 (blastopore) becomes the anus, while the mouth is formed 

 by an involution of the ectoderm of the anterior end of the 



1 Lankester, " On the Coincidence of the Blastopore and Anus in Paludina 

 vivipara." {Quarterly Journal of Microscopical Science, 1876.) 



