TYPE PLATYHELMINTHES. 



133 



anterior margin of the body, which is supposed to be tactile 

 in function. 



No excretory apparatus has as yet been described for the 

 Accela, but a reproductive system 

 with some interesting peculiarities 

 occurs. The male apparatus consists 

 of numerous spherical testes () whose 

 ducts unite to two vasa deferentia, 

 dilating below to form the seminal 

 vesicles (vs) and uniting in the mus- 

 cular iutroniittent organ. The female 

 organ is, however, relatively simple, 

 consisting of two club-shaped ovaries 

 (ov) whose short oviducts open almost 

 directly to the exterior near the pos- 

 terior end of the body by a pore ($ 9 ) 

 common to both male and female ap- 

 paratus. There is no vitellarium, no 

 shell-gland, no seminal receptacle, 

 and no special uterus, a state of 

 affairs indicating great simplicity of 

 structure compared with what is 

 found in the other orders. 



2. Order Alloioeoela. 



FIG. 71. DIAGRAM OF AN 

 ACCELOUS TURBELLARIAN 



(after VON GRAFF). 



The members of this order are 

 marine with the single exception of 

 Plagiostoma lemani, which is found in 

 the deep waters of the Swiss lakes. 

 They present a distinct advance upon 

 the Acoela in that a well-defined diges- 

 tive tract is present (Fig. 69, !>}, the 

 interval between it and the peripheral 

 musculature being completely filled up by the usual paren- 

 chyma and the organs imbedded in it. These forms are then 

 triploblastic, possessing well-defined ectoderm, rnesoderrn, and 

 endoderm, a condition found in all the higher orders. 



The mouth varies somewhat in position, lying either near 



e = eye. 

 m = mouth. 

 ot = otocyst. 

 ov = ovary. 

 p = parenchyma. 



t = testis. 



vs = vesicula seminalis. 

 $ = reproductive orifice. 



