THE TURBELLARIA 



In a number of Acotylea the egg duct is continued backwards 

 as a blind sac beyond the point of entrance of the uterus (Fig. 

 XVIII.); and in Trigonoporus this " accessory sac " effects a com- 

 munication with the exterior behind the female pore (compare 

 " Laurer's canal " in the Trematoda). 



It is worthy of note that both Lang (in Gunda) and von Graff 

 (in Planocera simrothi) have described the development of ova from 

 the lining of the intestinal epithelium ; this would go a long way in 

 support of the very close relations between the Turbellaria and 

 Coelentera, and of the view that there is no definite coelom in 

 the former group, it being represented by the intestinal caeca. 



Reproduction. The eggs of the Polycladida are not laid in 

 groups in capsules as in freshwater forms, but numbers are de- 

 posited in a jelly-like case, somewhat like the spawn of Nudibranch 



Fio. XVIII. 



1. Diagrammatic longitudinal sec- 

 tion of the terminal parts of the genital 

 ducts of Leptopla.ua. a, male pore 

 leading into the antrum masculinum, 

 into which the penis (5) projects ; c, 

 vesicula grauulorum ; <l, ductus ejacu- 

 latorius; e, the seminal vesicle; /, 

 sperm duct ; g, female pore leading 

 into the antrum femininum ; h, ootype 

 surrounded by shell glands ; j, acces- 

 sory sac ; k, oviduct. 



2. Similar diagram of Trigono- 

 porus. Letters as before. Here the 

 accessory sac (j) has effected a com- 

 munication with the exterior (/), so 

 that there are three genital pores. The 

 region of the body wall between g and 

 I is folded, and acts as an organ of 

 fixation. (Both after Lang.) 



Molluscs ; there is no yolk, but each egg has its own shell, which 

 may be operculated (Fig. XIX.). Some Polyclads undergo direct 

 development ; others pass through a free-swimming larval stage, 

 which was first noted by Joh. Miiller (48). 



The development of Discocoelis and others has been carefully 

 worked out by Lang. Segmentation is holoblastic, but unequal, 

 giving rise to micromeres and macromeres ; the mesoblast is very 

 early marked out as cells intermediate in size between the micro- 

 meres and the macromeres ; the latter do not directly become the 

 hypoblast, but cells free from yolk are budded off from them, which 

 gradually surround the centrally placed yolk masses. Meanwhile, 

 epibolic invagination has led to the formation of a definite embryo 

 with ciliated epiblast ; the central yolk masses are now devoured 

 by the hypoblast cells, to which they stand in relation of parent to 

 children, and the enteron gradually acquires a lumen, and effects a 

 communication with the exterior by means of an anteriorly placed 



