450 PHYSIOLOGY OF THE INVERTEBRATA. 



Projecting from tlie vas deferens is the flag or flagellnm. 

 The vagina and the male aperture (at the end of the penis) 

 open into a common cloaca, which bears filamentous gland- 

 ular appendages or mucous glands. These secrete albumin. 

 The muscular dart-sac (which contains a "calcareous" or 

 "' chitinous " dart or rod *) also opens into the common 

 cloaca. In connection with the female genital aperture, there 

 is a receptaculum seminis or spermatheca. Although Hdi.c 

 is hermaphrodite, cross-fertilisation takes place. When two 

 individuals copulate, the dart or spiculum amoris is protruded; 

 and no doubt acts as a stimulating organ. The dart is usually 

 broken up during copulation, but is afterwards replaced. 

 During coition, the semen of one individual, after being dis- 

 charged, is stored in the spermatheca of the other individual. 

 The ova are impregnated in the duct, and are invested in 

 albumin, which is enclosed within a calcified chorion. The 

 development is direct, and the young is hatched in the form 

 of the adult. 



" The Branchiogaster'opoda fall into two distinct series, of 

 which the one is hermaphrodite (the genital gland being an 

 ovotestis), and invariably opisthobranchiate ; while the other 

 is unisexual, and usually prosobranchiate." 



In some genera the penis is not developed (e.g., MurcMsonia, 

 Pleurotomaria), while in others the organ is developed (e.g., 

 Natica, Turritella, Volvia, Cyprcca). 



The Ptcropoda are hermaphrodites, provided with an ovo- 

 testis, which develops spermatozoa and ova. As in Helix' 

 these two generative elements are not mature at the same 

 time. The ovotestis has a single duct, the termination of 

 which may be provided with a receptaculum seminis, and 

 connected with a penis. Cross-fertilisation takes place, and 

 the ovum gives rise to young, provided with a rudimentary 

 shell, velum, and probably an operculum. In some forms 

 (HgalaxC) the shell is retained, while in others (Clio) it dis- 

 appears. 



* In some species the dart contains silica. 



