452 PHYSIOLOGY OF THE INVERTEBRATA. 



vesicula seminalis, at the termination of which is the prostate. 

 The prostate forms the spermatophores, which are discharged 

 into a receptacle (Fig. 79). The receptacle or sperm-sac 

 then leads into a muscular penis, at whose extremity is 

 situated the genital aperture. " The projection of the sperm- 

 sac occurs at the moment when, during coition, the sperma- 

 tophores pass from the penis of the male into the sac of the 

 mouth of the female. A true intromission of the penis into 

 the female genital opening appears impossible in these 

 animals, so that coition consists only in a simple juxtaposition 

 of the genital organs." (Von Siebold.) 



In the female Seipia, (Fig. 80) the genital organ consists of 

 an ovary, with an oviduct which opens near the anus. The 

 oviduct presents an enlargement called the oviducal gland. 

 In addition to these, there are nidamental and accessory 

 glands. These glands secrete a substance which invests the 

 ova as they pass from the oviduct, and which serves to glue 

 them to foreign bodies. The fecundated ovum undergoes 

 partial segmentation ; but for the further development of the 

 embryo, the reader is referred to Professor Lankester's paper 

 in the Quarterly Journal of Microscopical Science, 1875 ; 

 Grrenacher's paper in the Zeitsclirift fur Wissenschaftliche 

 Zoologie, 1876; and Mojsisovics's GepJialopoden der Mediier- 

 I'anen. 



The Tunicata. 



These animals are hermaphrodites; but cross-fertilisation 

 takes place as the spermatozoa and ova reach maturity at 

 different times. The testes and ovaries are racemose glands, 

 which usually lie among the viscera in the hinder portion of 

 the body. In many simple Ascidians, however, these organs 

 are situated in the lateral walls of the atrium, into which 

 their ducts * open more or less in the vicinity of the anus. 

 The impregnated ovum becomes completely segmented, and 

 passes through the morula and gastrula stages. Afterwards 



* The ovaries and testes of Appendicularia are devoid of ducts. 



