THE STRUCTURE OF XENIA HIOKSONI. 283 



tentacles have reached a length of -34 mm., and the tip of each 

 tentacle is distinctly trilobed when seen from the outer side, 

 i.e. there is an indication of the formation of the first two 

 pinnules, one on each side of the axis of the tentacle. 



From this point onwards the formation of pinnules takes 

 place regularly as the tentacles increase in size, as may be 

 seen from the table below. When the polyp (fig. 6) has 

 reached a length of 1'42 mm. the tentacles, which are "6 mm. 

 long, are seen from the outer aspect to bear four pinnules on 

 each side. If one of the tentacles be examined from the inner 

 side it will be seen that an inner row of pinnules has already 

 been formed (fig. 6a). The polyp from which fig. 7 was 

 drawn was 2'27 mm. long, and its tentacles 1'30 mm. long. 

 Seen from the outer aspect the tentacles show eight pinnules 

 on each side. For a distance of about two thirds of a milli- 

 metre from the tip of the tentacle, there are on the inner face 

 two rows of pinnules on each side of the middle line, but a 

 little nearer the base of the tentacles the three rows of pinnules 

 on each side, characteristic of the tentacles of the adult, may 

 be seen (fig. 7a). 



After the earliest stages of growth are passed the polyps 

 grow quite regularly, and the length of the tentacles bears to 

 the total length of the polyp an almost constant proportion. 

 From the appended table of measurements it will be seen the 

 tentacles form rather more than two fifths of the whole length 

 ot the polyp. With the increase in length of the tentacles 

 there is a corresponding increase in the number of pinnules 

 which the tentacles bear, and, as the following table shows, 

 there is a perfect series of examples, beginning with the very 

 young specimens, in which the tentacles are devoid of pinnules, 

 and ending with the largest polyps, whose tentacles show from 

 the outer aspect nineteen or twenty pinnules on each side of 

 the middle line. The gradual transition from the youngest to 

 the oldest polyps shows that the small individuals in this 

 colony are undoubtedly young buds in various stages of 

 development. They are all ''polyps/^ using the word in the 

 sense in which it was used by KoUiker. 



