212 PLATYHBLMINTHES. 



sense marked off from it (except in the case of the ripe generative 

 cells), and passing perfectly gradually into it. Cell limits, save for 

 the ripening generative cells, and possibly here and there a wandering 

 cell, are entirely absent from it. It should also be mentioned that 

 spaces or vacuoles filled with fluid are present in all parts of the 

 parenchyma. The parenchyma of the other Turbellaria, generally 

 speaking, resembles that of the Acoela in structure, but differs in 

 the fact that the organs enteron, cerebral ganglion, gonads are 

 more completely delimited from it- 

 It cannot be said that there is a perivisceral cavity in the Turbettaria. 

 There are vacuoles in the parenchyma, and these, in some Rhabdococlida, are 

 so much developed round the enteron as to suggest a body cavity. According 

 to v. Graff indeed, there is in such cases a lining of endothelium. 



As to the homologies of the parenchyma tissue, it appears probable, from 

 its condition in the Acoela, that it is a part of the endodemi in which the 

 enteric cavity is either not developed or lias collapsed, or become filled with 

 plasmodial extensions of its protoplasmic walls, of a nature similar to the filling 

 up of the enteron described by Bourne in a ooral polyp (Q. J. M. S., 28, p. 29). 



In the Acoela this absence of enteron extends to the whole of the endoderm. 

 The close relation of the parenchyma to the endoderm is not only suggested by 

 the condition in the Acoela, in which they are actually continuous, but is 

 indicated by an observation of Lang's that the endodermal lining of the enteron 

 contains extensions of the excretory system. This suggests that possibly the 

 continuity between the parenchyma and the endoderm in the Triclada, at any 

 rate, is closer than is supposed. The origin of the generative cells in the 

 parenchyma, and their passage through the vacuolatecl spongy mass to the 

 genital opening is exactly what must happen in Bourne's coral polyp with 

 a spongy enteron. At the same time I do not wish to suggest that the paren- 

 chyma ever had a continuous digestive space. I would rather suggest that the 

 Acoela are connecting forms between large Infusoria and the higher animals, in 

 which the endodermal protoplasm, though without a continuous cavity, is 

 partly differentiated into a number of important organs. 



FIG. 167 Planaria polychroa creeping with outstretched pharynx. 



The mouth leads into a muscular pharynx, which is contained 

 in a sheath, and can usually be protruded like a proboscis (Fig. 167). 

 The enteron, into which the pharynx opens, consists only of a simple 

 chamber (Fig. 168), or of a central chamber prolonged into branches, 

 which may themselves branch and even anastomose. In some cases 



