38 A Guide to the Zoological Collections in the 



PHYLUM I OF THE CCELOMATA. 



PLATYHELMINTHES. 



[5Ke0tcrn mM-t^u 46]. 

 The Platyhelminthes include four natural groups, 

 namely, (i) the Turbellaria, (2) the Trematoda, (3) the 

 Cestoda^ and (4) the Nemertina. Of these the Cestoda and 

 Tretnatoda are entirely parasitic, and are therefore much 

 degraded in form, the alimentary canal being either absent 

 or incomplete. The only characters that are common to 

 all four groups are that the body is flat and more or less 

 elongated and solid, and that the nervous system is concen- 

 trated in a mass or " ganglion " at the head end. Repro- 

 duction is usually sexual, but may sometimes, especially in 

 the larval stages, take place asexually. 



I. PLATYHELMINTHES TURBELLARIA. 



[®£stertt ®aU-ra0£ 46]. 



The Turbellaria are small Platyhelminthes with a leaf- 

 shaped body: some live in the sea, some in freshwater, and 

 some in damp places on land. 



The main features of their organization are illustrated 

 by enlarged drawings of Planaria polychroa and Planaria 

 lactea and of Cycloporus papillosus (Case 46), in which 

 the body-cavity is seen to be filled with parenchyma and the 

 intestine to end blindly. 



2. PLATYHELMINTHES TREMATODA. 



[SJUestcrn ® all-case 46]. 

 The Trematoda, or Flukes, also are small Platyhel- 

 minthes usually with a leaf-shaped body. They are para- 

 sitic, commonly in the alimentary canal of warm-blooded 

 vertebrates, whence they sometimes pass into the portal 

 vein and its affluents : other forms are found in the gill- 

 chambers of fishes, in the bladder of the frog, etc. In order 



