98 PLATYHELMINTHES [CH. 



tion of these glands forms the egg-cases in which one egg and many 

 yolk-cells are enclosed. As the egg-cases are formed they pass into 

 two great sac-like diverticula of the atrium, one situated on each 

 side of the body, called the uteri, i.e. "wombs." In these they are 

 carried about by the animal for some time, but are eventually laid, 

 and become attached to water-plants by the stickiness of their out- 

 side layer. There are two kinds of these egg-cases in Mesostoma, 

 one thin-walled, called " summer eggs," and the other thick-walled, 

 called "winter eggs." The former are believed to contain ova 

 fertilised by the spermatozoa of the same individual ; these develop 

 rapidly, devouring the surrounding yolk-cells and the resulting 

 young hatch out in April and May. These when they arrive at 

 maturity cross-fertilise one another, and as a result the thick-walled 

 capsules termed "winter eggs" are produced, which lie dormant 

 during the winter, whilst the parent turns opaque, sinks to the 

 bottom of the water and dies. In the spring young are hatched 

 from the winter eggs, which produce when mature summer eggs, and 

 in some cases are supposed after laying these to live on and produce 

 the winter eggs of the next season ; but in this respect the various 

 species probably differ from one another. 



The Turbellaria are a large group, and fall naturally into two 

 main divisions, viz., the Rhabdocoelida with a rod-like gut (Gr. 

 pa/38o9, a staff) and the Dendroeoelida with a branched one 

 (Gr. SeVS/ooi/, a tree). Each of these divisions is again subdivided; 

 thus the order Rhabdocoelida includes the sub-orders ACOELA, 

 ALLOIOCOELA, and RHABDOCOELA, whilst the Dendrocoelida are 

 divided into TRIGLADA and POLYCLADA. 



To turn first to the divisions of the Rhabdocoelida, the sub- 

 order ACOELA includes extraordinary forms in which there is no 

 digestive cavity ; the alimentary canal is represented by a porous 

 mass of endoderm cells, amongst the interstices of which the 

 digested food soaks. The endoderm completely fills the space 

 surrounded by the ectoderm and muscle layers; there is no 

 parenchyma. In almost every case there is a muscular pharynx 

 by which the animal adheres to its prey, and through which a, 

 pseudopodium-like mass of endoderm is protruded. This protrusion, 

 secretes a solvent which disintegrates the victim, and it then 

 engulfs the product after the manner of an Amoeba. In some 

 cases, however (Convoluta), the endoderm is infested with small 

 green Algae, and the animal lives largely on the compounds formed 

 by these, needing only a scanty diet of Protozoa and Diatoms to- 

 supplernent its internal provision. 



