114 



INVERTEBEATA 



CHAP. 



(Fig. 90) and finally disappear altogether, so that the larva gradually 

 becomes transformed into a Polyclade worm. 



Of the internal changes which meanwhile take place the most 

 noteworthy is the formation of the adult pharynx. This arises as a 

 ring-shaped evagination of the innermost part of the larval stomo- 

 daeum {stom, Fig. 91), lined by thin cells, from the bottom of which the 

 pharynx arises as a ridge-like thickening (pA, Fig. 91). During larval 

 life this evagination is virtually a closed cavity which communicates 



Fig. 91. — Median sagittal longitudinal section through larva of Yungia aurantiaca. 



(After Lang.) 



end, endodermic sac ; end.ant, anterior diverticulum of gut passing over brain ; d.musc, dorso- 

 ventral muscles ; g, brain ; l.musc, larval muscles ; ph, rudiment of adult pharynx ; sJh, pharynx-sheath ; 

 stom, stomodaeum or larval pharynx ; rh, rhabdites in ectoderm-cells. 



with the stomodaeum by the narrowest slit. As the adult condition 

 is attained, the outer part of the larval stomodaeum is everted and 

 its walls become parts of the external surface of the body, the 

 evagination alluded to above opens widely into the inner portion of 

 the stomodaeum and becomes the adult pharyngeal sheath surrounding 

 the pharynx (Fig. 92). During larval life muscles become differentiated 

 from cells of the parenchyma, and some of these (l.musc, Fig. 91) which 

 serve to elevate and depress the ciliated processes are of a provisional 

 character, whereas others (d.musc, Fig. 91) persist as the dorso-ventral 

 muscles of the adult. The endodermic sac develops an anterior pouch 



