4i8 PHYSIOLOGY OF THE INVERTEBRATA. 



the larva is vermiform, and Has no skeleton ; in tlie Eclii- 

 nidea and the Ophmridea it becomes pluteiform, and develops 

 .a special spicular skeleton." 



The Trichoscolices. 



The Turbellaria multiply by two methods, (a) transverse 

 fission; (h) by sexual organs. In the smaller Bliahdocoda, 

 the first mode of reproduction is the rule, for no genital 

 •organs are present. These animals are both monoecious and 

 ■dioecious ; and the " genital and copulatory organs of both 

 sexes are situated upon one and the same individual so that 

 they are capable of self-impregnation ; but there is generally 

 a reciprocal copulation." * 



In the higher Turbellaria the female genital organs consist 

 •of the following parts : a germarium which develops ova ; 

 a vitellarian gland in which the vitellus or food yolk is 

 formed ; an oviduct ; a uterus and vagina ; and a sperma- 

 theca — ^the function of which is to store the semen after the 

 act of coition. The male genital organs consist of a testis, 

 vas deferens, and penis ; the latter " is often eversible and 

 •covered with spines." The impregnated ova are enclosed 

 within a hard shell. In some genera the hard shell is only 

 formed on the winter ova, while the summer ova are only 

 covered by a soft vitelline membrane. The rhabdoccelous 

 ovum undergoes complete segmentation and the embryo 

 passes directly into the adult form. 



In the marine Planarice, there is no vitellarium, and in 

 some of these animals the embryo after leaving the ovum 

 differs considerably from the adult. The Planarice are 

 hermaphrodites, but Pldnaria dioica is unisexual. The Proc- 

 ttuilm are nearly always hermaphrodites, and the ova and 

 spermatozoa are discharged externally by the dehiscence of 

 the integument. 



Nemertes is dicecious, and the genital organs ( $ and 9 ) 

 have the same structure, being sacs filled with spermatozoa 

 * Coition has been observed in Planaria, Mesostomum, &c. 



