THE STRUCTUEE OF XBNIA HIOKSONI. 293 



of the storaodseum and partly by a small plug of mucus. The 

 siphonoglyph is not yet differentiated. The distance from 

 the mouth to the inner end of the ccelenteron is about "7 mm. 

 The mesenteries may be traced almost to the inner end of the 

 coelenterouj and their retractor muscles are just distinguish- 

 able. The dorsal mesenteries are slightly thicker than the 

 others, but their mesenterial filaments have not yet been 

 formed. The ccelenteron is formed by enlargement of the 

 superficial endodermic canal. An evagination of the outer 

 wall of the canal forms the ccelenteron of the free portion of 

 the polyp, and pushes outwards the mesogloea and ectoderm, 

 thus giving rise to the protuberance which forms the free 

 portion of the young polyp. A diverticulum produced by the 

 bulging inwards into the mesogloea of the inner wall of the 

 canal forms the inner part of the ccelenteron. 



The endoderra of the outer wall of the superficial canal is 

 much thicker than that of the inner wall (fig. 29). This dif- 

 ference enables one to see in section (fig. 28) that the endo- 

 derm of the whole of the free portion of the polyp is derived 

 from the thick outer wall of the canal. The endoderm of the 

 portion of the ccelenteron situated in the outer part of the 

 stem is formed directly from that of the canal, while the endo- 

 derm of the inner portion of the ccelenteron is derived from 

 that of the thin inner wall of the canal, which has grown in- 

 wards into the mesogloea. The ccelenteron gives off a large 

 canal at its base, which opens into the adjacent longitudinal 

 canals. During its short course in the stem the ccelenteron 

 communicates several times by means of short canals with the 

 neighbouring superficial and longitudinal canals. 



Among the endoderm cells of the middle portion of the 

 ccelenteron there are a very few flagella-bearing cells. The 

 flagellum is a short conical or finger-shaped process which in 

 these sections is never more than 15 fx in length. 



3. A slightly longer bud (No. Ill in the table, p. 284), '56 

 mm. long, possesses, like the preceding example, tentacles of 

 different sizes. In this specimen also the dorsal and ventral 

 tentacles are largest, the lateral ones being next in point of 



VOL. 42, PART 3. — NEW SERIES. D 



