THE PLATYHELMIA 



Tapeworms, and Turbellarians, which alone are now included 

 amongst the Platyhelmia, together with the probably degenerate 

 forms known as " Mesozoa." 



The Characters of the Platyhelmia. The animals belonging to 

 the three classes Turbellaria, Trematoda, and Cestoidea -while 

 exhibiting many differences in form, habits, and life-history yet 

 present certain fundamental points of agreement, so that it is 

 possible to picture a common ancestor from which the three groups 

 have been descended. Such an ideal Platyhelmirith would have 

 had a somewhat oval body, flattened from above downwards, and 

 with a distinct prostomium or region in front of the mouth and con- 

 taining the brain (Fig. II.). The surface of the body was probably 

 clothed with a ciliated epidermis similar to that of the Turbellaria 

 but of a simpler character, so that the animal was able to move 

 freely in the sea ; in this movement it was aided by the muscular 

 system which had developed below the epidermis with which it had 

 lost its connection and become arranged to form circular and 

 longitudinal sheets. No doubt the ancestral form was more or 

 less closely connected with the Coelentera by means of animals 

 of which we know nothing. (There are some features of 

 resemblance to the Ctenophora, as Lang has pointed out. 1 ) But 

 in the Platyhelminth the endoderm had become separated from 

 the ectoderm by a great development of mesodermal tissue filling 

 up the blastocoele, and consisting of vertically arranged muscle 

 cells and of a packing of peculiar connective tissue cells ; in this 

 compact mass of cells or parenchyma distinct generative and 

 excretory organs had become differentiated, having each its own 

 independent communication with the exterior. Nevertheless, no 

 definite space, or coelom, existed in the substance of this inter- 

 mediate mass of cells. The archenteron of the coelenterate 

 ancestor had, however, become separated into a metenteron and 

 a coelom, which is represented by the cavity of the genital organs. 



The excretory system consisted of a pair of laterally placed 

 canals, consisting of a series of perforated cells, some of which carry 

 cilia ; from each canal small lateral branches are given off which 

 branch and anastomose to form a network from which arise still 

 liner twigs, each of which terminates in a "flame cell" (Fig. I). This 

 flame cell (or pronephridiostome of Vejdovsky) is a comparatively 

 large, hollow cell from whose base, in which the nucleus is situated, 

 a bunch of long cilia projects into the cavity ; the flickering appear- 

 ance of a flame results from the synchronised movement of the cilia. 

 The cavity of this terminal cell communicates only with the excre- 

 tory tubule. It is possible that in the earliest ancestors a number of 

 isolated cells became hollowed out, and ciliated like this flame cell, 



1 This is discussed by Mr. Bourne in Part II. " The Ctenophora," pp. 16 et seq. 



