THE STRUCTURE OF XENIA HICKSONI. 217 



cylindrical or club-shaped, 2 mm. to 5 mm. in length, and *5 mm. to 

 1 mm. in diameter. The tentacles are eight small rounded lobes 

 arranged round the mouth ; they are never longer than "25 mm., 

 and never possess pinnules. 



Bourne provisionally referred this specimen to Kolliker's species 

 Heteroxenia Elizabeths, and, after comparing the specimen with 

 Kolliker's original account and figures (1874), 1 can confirm his con- 

 clusion. This specimen is much smaller than Kolliker's, its polyps 

 being only about half the size, but there are many points of agreement 

 between them, viz. general anatomy and proportion of parts, size, and 

 structure of zooids; size, colour, and distribution of spicules; canals, &c. 



After carefully examining the two kinds of small individuals (viz. 

 young polyps aud zooids), I have no doubt that they are entirely 

 different in nature. The external characters are quite different, and 

 there are important differences in their internal arrangements, to 

 which reference will be made. 



External Characters of the Young Polyps of Heteroxenia Elizabethce. — 

 The youngest polyp, which is also by far the smallest individual on the 

 colony, is "64 mm. in length (Fig. 38). Its tentacles are finger-shaped 

 lobes "24 mm. to "3 mm. in length. The tip of one of the tentacles is 

 slightly three-lobed, but all the other tentacles have simple rounded 

 ends. 



A polyp 1'4 mm. long, the tentacles of which have attained a length 

 of "4 mm., are trilobed at their ends, i.e., the first pinnule on each side 

 of the axis is being formed. The formation of pinnules proceeds regu- 

 larly with the growth in length of the tentacles, and the adult size and 

 characters are gradually attained From the appended table it will be 

 seen that there is a complete series of stages from the smallest polyp, 

 •64 mm. long, with simple tentacles devoid of pinnules, to the largest 

 polyp, nearly 30 mm. long, the tentacles of which show on the outer 

 face twenty-four pinnules on each side of the middle line. In this 

 Heteroxenia the ratio between the length of the tentacles and the total 

 length of polyp is not quite as constant as in the Xenia considered 

 above. The tentacles form about one-fourth to one-third the length 

 of the whole polyp. 



