PHYLUM PLATYHELMINTHES 



271 



becoming gradually longer and flatter, and the arms are 

 gradually reduced in length, till, eventually, they become com- 

 pletely absorbed. 



The development of the Triclads is very different from that of 

 the Polyclads. Each egg-capsule or cocoon encloses a number of 

 oosperms and a quantity of yolk-cells. After segmentation a 

 blastoderm is formed composed of a rounded mass of cells 

 surrounded by the yolk- material, the cells of which become more 

 or less fused. Around the periphery of the blastoderm a layer 

 of cells forms a thin membrane the ectoderm. About the 

 middle appears a rounded group of cells which passes to the 

 periphery and becomes connected with the ectoderm ; a cavity 

 is formed in its interior and it becomes converted into the 

 embryonal pharynx. Mean- 

 while a group of four cells 

 enclosing a cavity is modified 

 to form the foundation of the 

 endoderm, and increasing in 

 number gives rise to the em- 

 bryonal intestine, into which 

 the embryonal pharynx soon 

 opens, the latter opening on the 

 exterior by a mouth aperture. 



The embryonal pharynx (Fig. 

 222, ph } ) has the function of 

 swallowing the yolk-matter with 

 which the embryonal intestine 

 becomes greatly distended. At a subsequent stage the embryonal 

 pharynx and intestine are aborted, and the former comes to be 

 represented merely by a mass of cells. In this a cavity arises 

 -the cavity of the permanent pharynx. The permanent intestine 

 becomes formed and the cavity of the pharynx opens into its lumen. 

 Subsequently the permanent mouth makes its appearance. The 

 brain is formed in the thickness of the blastoderm, and thus 

 appears to be of mesodermal origin, not of ectodermal, as in the 

 Polyclads. 



In some Rhabdocoeles (certain species of Mesostoma) two distinct 

 kinds of eggs are formed summer and winter eggs. The oosperms 

 are deposited singly, and each, together with a mass of yolk-cells, is 

 enclosed in a chitinous, usually stalked, shell. Segmentation 

 takes place very much as in the Polyclads. No embryonal 

 pharynx is formed, the permanent pharynx performing the function 

 of swallowing the yolk-material : it appears to be of endodermal 

 derivation. The intestine arises from a group of cells the 

 primitive endoderm among which a cavity appears. But in 

 some Rhabdocoeles no definite intestinal epithelium is developed, 

 and the syncytial mass which represents it is only to be dis- 



FIQ. 221. Miiller's larva of 



Lateral view. 2-6, ciliated processes. 

 (From MacBride, after Lang.) 



