I" PLATODESr— GENERAL REMARKS 135 



p. 164), without hooks on the rostelhim, with over 1000 proglottides, sexual apertures 

 placed on the edge ; in the human intestine. The finn, <i Cystkerms, lives in the 

 muscles of the ox. T. solium (Fig. 117, B, p. 164), with double circle of hooks on the 

 rostellum. Sexual apertures placed on the edge. About 800 proglottides. Finn : 

 Cysticerdis cellulosm in flesh of the pig (Fig. 120, p. 172). T. sermta, in intestine 

 of the dog. Finn : Gysticercus pidformis, in the liver of rabbits and hares. T. crassi- 

 coUis, m the intestine of the domestic cat and of other Felida;. Finn : Oysticsrcus 

 fasciolaris, in the liver of the mouse. T. cucumerina, in intestine of the dog, and 

 scolex-like finn in the body cavity of the dog-louse. T. cximrus, in intestine of the 

 dog. Finn : Cmnurus cerehralis, in brain and spinal marrow of the sheep, causing 

 "staggers." T. echinococcus, in the small intestine of the dog; finn, HcJdnococcus 

 veterinorum in the liver of man and in that of difi'erent domesticated ungulates. 



I. General Remarks. 



The race of the Platodes forms a very natural division of the 

 animal kingdom, containing the three classes of the Turbellariu, the 

 Trematoda, and the Cestoda. Their fullest development is shown in the 

 organisation of the free living Turhellaria, which move forward by 

 creeping or gliding, while in the Trematoda and Cestoda many degenera- 

 tions and simplifications have taken place in consequence of parasitism. 

 The organisation of the Turhellaria more than any other therefore 

 supplies us with the clue to understand the systematic position of the 

 Platodes and their aflEinities. 



Of all Metazoa it is the Platodes, and especially the Turhellaria, 

 whose general structure most agrees with that of the Ccelenterata, i.e. 

 of the Cnidaria. They might almost be placed near the Gtenophora 

 as creeping Cnidaria. 



On account of the absence of a ecelome between the alimentary 

 canal and the integument, and of a separate blood-vascular system, 

 the function of circulation is performed, as in the Cnidaria, by the 

 digestive system as gastro-canal apparatus. An anus is wanting here 

 as there. The Platodes show, however, on the other hand, many 

 essential differentiations of organisation which we miss in the Cnidaria, 

 and which are in great part to be referred to the creeping mode 

 of life. All the Platodes are bilaterally symmetrical; we can dis- 

 tinguish in their bodies an anterior and posterior, an upper and lower, 

 a right and left. The aboral surface of the body of the Cnidaria 

 becomes in the Platodes the dorsal surface, the oral the ventral 

 surface, in whose centre the mouth originally lies. The sensory organs 

 collect chiefly at that part of the body which goes first in creeping, 

 i.e. at the anterior end, and the principal part of the central nervous 

 system also, the brain, which originally lay at the aboral pole, i.e. at 

 the centre of the dorsal surface, has, following the sensory organs, 

 moved more or less far forward. 



Those organs and systems of organs which, in the Ccelenterata, 

 showed the tendency to leave the body epithelium and deposit 

 themselves below it, forming a middle layer, have in the Platodes 

 become markedly mesodermal, viz. (apart from the connective tissue) 



