THE STRUCTURE OF XENIA HICKSONI. 223 



siphonoglyph is not yet differentiated. The distance from the month 

 to the inner end of the coelenteron is about "7 mm. The mesenteries 

 may be traced almost to the inner end of the coelenteron, and their 

 retractor muscles are just distinguishable. The dorsal mesenteries are 

 slightly thicker than the others, but their mesenterial filaments have 

 not yet been formed. The coelenteron is formed by enlargement of 

 the superficial endodermic canal. An evagination of the outer wall 

 of the canal forms the coelenteron 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 coelenteron. 



The endoderm of the outer wall of the superficial canal is much 

 thicker than that of the inner wall (Fig. 29). This difference enables 

 one to see in section (Fig. 28) that the endoderm 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 coelenteron situated in the 

 outer part of the stem is formed directly from that of the canal, while 

 the endoderm of the inner portion of the coelenteron is derived from 

 that of the thin inner wall of the canal, which has grown inwards into 

 the mesogloea. The coelenteron gives off a large canal at its base, 

 which opens into the adjacent longitudinal canals. During its short 

 course in the stem the coelenteron 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 coelenteron 

 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 ju in length. 



3. A slightly longer bud (No. III. in the table, p. 215), '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 size, and the four intermediate ones 

 somewhat smaller. Does this indicate the order of development of 

 the tentacles 1 This bud resembles the preceding one in all essential 

 particulars. 



4. There are several important new structures noticeable in a young 

 polyp -8 mm. in length (No. IV. in table, p. 215). The tentacles are 



