426 



INVERTEBRATA 



CHAP. 



from the regions of the cuticular bands, and form the " wheel-organs " 

 of the adult (-Fig. 337, B). They eventually become connected with 

 each other by a very narrow upper lip, which runs above the mouth 

 and beneath the proboscis in such a manner that the latter is excluded 

 from the trochal field. The dorsal feeler, or antenna, is formed at 

 the spot where the first rudiment of the brain still retains a 

 connection with the ectoderm. 



The muscles all appear to be of ectodermal origin. Those first 

 observed by Zelinka were longitudinal fibres running from the 

 " wheel-organs " to the middle of the body. Circular muscles are 



caud 



stom 



end 



FIG. 337. Ventral views of two embryos of Callidina russeola at a stage not long 

 before hatching. (After Zelinka.) 



A, younger stage: stage nf invagination of rudiments of wheel-organs. B, older stage : slage of 

 evagiiiaticm of rudiments of the wheel-organs and of preponderant growth of the tail, if, anus ; nvuil, 

 tail; I'ltixLf, caudal fork; end 1 , endodennal stomach; /'/'<>'<, rudiment of proboscis ; stum, secondary 

 stomodaeum ; tr, cutifular band marking place where cilia of the "wheels" will appear later. 



developed later from ectoderm cells, at regular intervals ; these cells 

 retain their plasma and their individuality, in contradistinction to 

 most of the other ectoderm cells which flow together and form a 

 thin syncytiuin, the so-called " hypodermis." 



The first traces of the excretory organs appear comparatively 

 later as two streaks of dull glistening cells, situated at the sides of the 

 pharynx. Subsecpaently they are developed into the coiled canal 

 and adherent soleuocytes, of which in the adult there are eight on 

 each side. The bladder into which they open is derived from the 

 endoderni contained in the foot region ; this endoderm has been pushed 

 forward by the development of those ectoderm cells which constitute 

 the foot gland. 



