Introduction to Animal Morphology. 159 



(Dendrocoela). The female organs consist of — ist, a 

 ^erm-gland secreting the central part of the Q^^; 

 2nd, one or two yelk-glands secreting a cellular ma- 

 terial, not homologous with true yelk, which contri- 

 butes chiefly to the formation of the embryo. These 

 may open separately or in common into the oviduct, 

 which is at one part dilated into a uterus, where the 

 eggs accumulate and begin to develop. To this is 

 added a shell-secreting gland and an external duct or 

 vagina, often with a dilatation for the reception of the 

 semen (spermatheca). The male organs in Rhabdo- 

 coela consist of a pair of pouch-like testes, with ducts 

 opening in seminal vesicles, and ending in a perforate 

 cirrus or penis, armed with recurved hooks, and, 

 when retracted, contained in a sac. In Dendrocoela, 

 many vesicular testes exist scattered through the 

 body, sometimes in pairs (Bipalium). In Convoluta, 

 some individuals have developed male and rudimental 

 female organs, and others the_ reverse, leading us to 

 the dioecious forms (Microstomidae, Acmostomum 

 dioicum, Nemertinea, Planaria dioica). The sex- 

 organs of Nemerteans are simple follicles between the 

 intestinal caeca, opening laterally by pores above the 

 lateral nerves. The spermatozoa are rod-like. The 

 ova are rapidly developed ; sometimes several embryos 

 arise from one Q'g^. In some cases (Borlasia), the 

 stomach cavity appears to formTby invagination 

 [Hubrecht). In other Nemerteans, the^ embryo forms 

 as a vesicle of one layer (blastula), which becomes 

 ciliated, free, and then by invagination becomes a 

 gastrula,* In Nemertinea, the soft-shelled eggs are 



* The blastula stage precedes the perfect triploblastic planula stage in 

 Ascidians, Amphioxus, &c., as well as in Nemerteans [Salensky). Knappeu 

 describes Planaria as a true diblastula. 



