4 W. G. RIDEWOOD. 
The range in the number of pinnules of the plumes in the same species, and even in 
different plumes of the same individual, is too wide to allow of such charactcr being 
utilised for purposes of discrimination. Although the two plumes of the males of 
C. sibogae have no pinnules, those of the neuter individuals are provided with pinnules 
resembling those of other species. The massiveness of the axes of the plumes of 
C. nigrescens is distinctive of that species, but as regards the form of the apex it is not 
easy to say whether in C. nigrescens the apex is swollen or not, it varies so much with 
the different degrees of extension and contraction of the plumes (cf. figs. 23 and 24, 
plate 5). Terminal bulbs with refractive colourless beads such as occur in C\ dodeca- 
lophus (text-fig. 1) are met with in C. hodgsoni (figs. 31 and 32, plate 5), but there are 
no such swellings in the polypides of C. levinsenz, in the neuter polypides of C. sibogae, 
nor in the adults of C. gracilis, although in the buds of the last species they occur on 
the first and sometimes on the second and third plumes, and occasionally persist in the 
adult stage (10, p. 20). In males of C. sibogae the refractive beads occur along the 
whole course of the two pinnule-less plumes. 
Text-Ficurr 1.—Cephalodiscus dodecalophus ; plumes seen from the aponeural aspect and from the side. 
The size of the body of the polypide varies somewhat with the condition of the 
gonads, but it is fairly uniform for the same species, and would be a useful character if 
one could estimate the bulk or the weight of the body. In consequence, however, of 
the great mobility of the parts, and their capacity for expansion and contraction, the 
linear measurements are not very reliable, and the length of the body of C. levinseni 
and C. nigrescens may be an excessive index of the bulk of the body in consequence of 
the limitation imposed upon the width of the body by the narrowness of the tubular 
cavity in which the polypide dwells. C. dodecalophus and C. hodgsoni, on the other 
hand, being less restricted, have a body less cylindrical. 
The measurements found to be most reliable are those from the front of the buccal 
shield to the posterior end of the visceral mass. The length of the body of C. dodeca- 
lophus is usually given as 1 mm., but it is not explained how this measurement is 
taken ; presumably one is expected to measure from the anus to the end of the visceral 
mass, but even then the recorded measurement falls short of the actual size. The 
average length of the polypide from the front of the buccal shield to the end of the 
