376 h. DONCASTER. 



of the originally paired head cavities. At this stage the 

 head-coeloni is clearly separated, from the hood cavity, but 

 later the latter seems to be obliterated, and the coelora extends 

 into the base of the hood. 



The alimentary canal of the head has now a svell-defined 

 cavity, which extends back into the neck ; in its walls cell 

 boundaries can no longer be distinguished, and they appear 

 as continuous masses of tissue with nuclei at intervals. 



The latest sections made were of ninth-day larva? (PI. 20, 

 fig. 27); these show the same structures as are described 

 above, but are further advanced, and approach more nearly 

 the adult condition. 



Before turning to the structure of the trunk, that of the 

 neck must be shortly described. When a series of sections 

 of the sixth day is followed backward, a little behind that 

 represented in fig. 26, a pair of oval cavities appear suddenly, 

 one on each side, between the pharynx and the epidermis 

 (fig. 28). These are the anterior ends of the trunk coelom, 

 which diverge a little in front (cf. the figures of the living 

 head), and are overlapped both dorsally and ventrallj^ by the 

 mesodermal structures of the head. Followed back, how- 

 ever, the latter disappear rapidly, and at the same time the 

 pharynx bends towards the ventral surface, so that the two 

 coelomic cavities, which take up the greater part of the 

 section, are separated dorsally by a thin mesentery. The 

 epidermis begins at this point to assume the vacuolated 

 structure described above, but this is, as a rule, not well 

 shown in sections owing to shrinkage; sometimes, however, 

 as in fig. 29, jj. t., it is well seen. A paired mass of nuclei is 

 also seen in the dorsal epidermis, which is the rudiment of 

 the "olfactory organ" (''corona ciliata," ol.). A little 

 further back the ectodermal pharynx joins the true gut, the 

 change being marked by the alimentary canal becoming very 

 narrow and lying like a septum from the dorsal to the 

 ventral surface, supported at each end by a short mesentery, 

 instead of being thick-walled and lying against the ventral 

 body-wall. 



