58 ESSENTIALS OF 200LOGY 



which form hydatid tumours in the liver of the sheep and 

 other animals. 



General Considerations. The parasitic Platyhelminths 

 have undergone great modification and degeneration in adapt- 

 ing themselves to their strange life-histories. Their structure, 

 therefore, is more interesting from the point of view of the 

 effects of parasitism than from the morphological. A con- 

 sideration of the free allies, the Turbellaria and the Nemer- 

 teans, shows that a blastula is formed and that endoderm 

 cells are developed at one pole, the vegetable pole, and may 

 indeed enclose a cavity communicating with the exterior, a 

 primitive enteron being produced, and the opening is termed 

 the blastopore. From the same region also a mass of cells 

 is budded off which intervenes between the endoderm and the 

 ectoderm and is resolved into the muscles and the mesenchyme. 

 This is the mesoderm. The blastopore, or the region which 

 corresponds to it, is carried inwards by the growth of the 

 ectoderm as the stomodeum, and forms the mouth and some 

 part of the anterior end of the alimentary canal. But, as has 

 been stated, unless in the Nemertinea, the alimentary canal 

 communicates only with the exterior by the mouth. The struc- 

 ture, then, is essentially that of the Coelenterates, an ectoderm 

 continuous with an endoderm at the mouth developed at the 

 vegetative pole of the egg. To these structures there has been 

 added a mesoderm between the two primary layers, and the 

 mesoderm is resolved into muscles and a highly developed 

 reproductive system. This mesoderm is also invaded by 

 ectodermal invaginations which act as excretory organs, the 

 protonephridia. In the case of the Nemertinea the mouth is 

 also developed from the blastopore, but the posterior end of 

 the intestine establishes an anal opening later. 



The primary layers of the Metazoon are the ectoderm and 

 the endoderm, and the Platyhelmia have interposed between 

 them a third layer, or mesoderm, which gives rise to organs 

 connected with reproduction, to muscles and to spaces in 

 which metabolic products circulate. The spaces represent a 

 primitive body cavity and may, as in the wheel animalcules 

 (Eotifera), and the larvae presently to be described, be so 



