160 THE ANATOMY OF INVERTEBRATED ANIMALS. 



provided with a second hollow muscular proboscidiform organ, 

 which may be termed the frontal proboscis. 



In all the higher rhabdoccelous Turbellaria, the female 

 generative apparatus becomes complicated by the presence 

 of a special gland, the vitellarium (Fig. 32, m), in which an 

 accessory vitelline substance is formed. There is a single or 

 double germarium (Fig. 32, £), having nearly the same struct- 

 ure as the ovary of Macrostomum, and the ova are formed 

 in it in the same way. When detached, however, they con- 

 tain no vitelline granules ; but the two vitellaria, which are 

 long and simple or branched tubes, open into the oviduct ; 

 and the vitelline matter which they secrete envelops the 

 proper ovum, and becomes more or less fused with it, as it 

 passes into the uterine continuation of the oviduct connected 

 with the outer, or vaginal, end of the uterus. There is usually 

 a spermatheca, or receptacle for the seminal fluid (Fig. 32, &), 

 and the eggs, after impregnation, are inclosed within a hard 

 shell (Fig. 32, n). The testes and vasa deferentia (Fig. 32, 

 d, e) generally have the form of two long tubes. The penis 

 is often eversible and covered with spines (Fig. 32, g). 



In some genera a difference is observed between the eggs 

 produced in summer, which have a soft vitelline membrane, 

 and those produced later. These so-called winter ova have 

 hard shells. 



The water-vascular system consists of lateral trunks, 

 which open by a terminal pore, or by many pores, and give 

 off numerous ramifications. They are not contractile, but 

 their inner surface is ciliated. 



Many of the Rhabdocoela multiply by transverse fission ; 

 and, in the genus Catenula, the incompletely separated ani- 

 mals produced in this way swim about in long chains. 



The vitellus of the impregnated ovum undergoes complete 

 yelk-divison, and the embryos pass directly into the form of 

 the parent ; but the precise nature of the steps of the devel- 

 opmental process requires further investigation. However, 

 there seems little reason to doubt that the ectoderm and en- 

 doderm are formed by delamination. 



In the remaining Aprocta, termed Dendrocoela, the diges- 

 tive cavity gives off many csecal, frequently branched, pro- 

 cesses into the mesoderm, one of which is always median and 

 anterior (Fig. 33) ; and the mouth is always provided with a 

 proboscis. Some (Procotyld) have a frontal proboscis, ai^d 

 others {Bdellura) a posterior sucker. The animals commonly 



