THE TURBELLARIA 35 



terminate blindly (Fig. XVI. 5) ; in Yungia some of these branches 

 open to the exterior ; while in Cydoporus such openings occur 

 close together around the entire margin of the body (Fig. XV. 

 7). This elaborate system of caeca is no doubt intimately related 

 to the great size of these Polycladida, for thereby nutriment is 

 conveyed throughout the body, so that the caeca function as a 

 lymphatic system. Nevertheless, there is but little structural 

 difference between the caeca and the stomach, either here or in the 

 Triclads, in which a similar process of extension, though to a less 

 degree, has taken place. Lang regards the enteric system as 

 homogenous with that of Ctenophora, from which he derives the 

 group ; he therefore uses the term " gastrovascular " system to 

 indicate the stomach and its caeca. How far this is true morpho- 

 logically is an open question, but to some extent the comparison 

 is true physiologically. 



The excretory system of the Polycladida is very insufficiently 

 known ; the pigmentation and large size of these forms are inimical 

 to study in the fresh state, in which alone the excretory system 

 can be with certainty made out ; nothing, indeed, is known as to 

 the position of the main trunks and excretory pores ; all we know 

 is that the general features of the system are similar to those of 

 other Platyhelminths. 



The nervous system appears to retain, to a great degree, 

 certain ancestral features ; it has, indeed, sunk below the epidermis 

 and dermal musculature ; but it retains the form of a close network, 

 in which cells and fibres take a share, extending all over the body. 

 This network, which is more strongly developed ventrally, in 

 relation to the more muscular character of this surface, radiates 

 from a brain which is more or less centrally situated, though in 

 longer forms it naturally occupies a more anterior position. Only 

 in Oligocladus is it behind the mouth. The nerve strands compris- 

 ing the network, however, are not all of a uniform size ; usually 

 three pairs of nerve tracts are stouter than the rest, viz. a pair 

 lying along each side of the median ventral line, a less conspicuous 

 strand along each lateral margin, and a pair of dorsal nerves. 



The eyes in this order have the same structure as in some 

 Triclads (Fig. XV. 6), but are usually very numerous ; their 

 position and arrangement are variable, and serve as useful 

 diagnostic family and generic characters ; there is always a 

 group on the brain, usually on the tentacles; or they may lie 

 along the margin of the body, 



The genital organs attain, in the Polyclads, a more diffuse 

 character than in the preceding orders, .both male and female 

 gonads being " follicular," and extending throughout the greater 

 part of the body. It seems that here too, as in the case of the 

 gut with which the organs are in close contact, this racemose 



