PLATYHELMINTHES 213 



in others, for example Planaria alpina, the eggs are only produced 

 during the winter months. Mesostomum produces two kinds of eggs 

 which are called ''summer" and "winter" eggs. The "winter" eggs 

 have a thick shell and are well supplied with yolk; they remain in the 

 uterus and escape only with the death of the parent. The "winter" 

 Qg^ can remain dormant for a long period. The "summer" egg is very 

 thin-shelled and has very little yolk. The development is very rapid 

 and the young embryos are seen moving in the uterus of the parent 

 seventy-two hours after the appearance of the eggs. They escape by 

 the genital pore and their formation does not involve the death of 

 the parent. The term "winter" and "summer" egg is not entirely 

 apposite, for "winter" eggs are often found in midsummer. The 

 "winter" egg is a method of carrying the species over unfavourable 

 conditions which may develop in winter or in summer. The ' ' summer " 

 egg is a means for rapid multiplication when conditions are favour- 

 able. 



Asexual reproduction occurs commonly in the Turbellaria. In 

 Microstoma lineare the hinder end buds off new individuals which re- 

 main attached for some time so that chains of three or four individuals 

 in different stages of development are often seen. Planarians undergo 

 autotomy, cutting themselves in two by a ragged line which traverses 

 the middle of the body. Lost parts are easily regenerated in the 

 Tricladida and the group is a favourite one for experimental work on 

 regeneration. 



Having thus provided the reader with a general account of the 

 organization of a platyhelminth it will now be possible for us to follow 

 the systematic arrangement of the phylum, to define the divisions and 

 to point out features of interest in various forms and life histories. 



Class TURBELLARIA 

 The Turbellaria may be defined as Platyhelminthes which are nearly 

 all free living and not parasitic, which retain the enteron; which have 

 a cellular, ciliated outer covering to the body; which usually have 

 rhabdites; and which do not form proglottides. Suckers are very 

 rarely present. 



The systematic arrangement of the Turbellaria is based primarily 

 on the structure of the gut. There are four orders : (i) Acoela, (ii) Rhab- 

 docoelida, (iii) Tricladida, (iv) Polycladida. 



Order ACOELA 



In these the gut is not hollow but consists of a syncytium formed by 

 the union of endodermal cells. There is no muscular pharynx. 

 Primitive features are the nerve net and the fact that the germarium 

 and vitellarium are not separated. Convoliita roscojfensis is the best 



