REPORT ON THE CRINOIDEA. 57 



A similar inequality in the development of the genital glands has been noticed by Prof. 

 A. Awassiz as oceurriuo; in the Echini/ 



This frequent difference in length between the anterior and the posterior arms of 

 Actinometra, accompanied by the difference in the character of their terminal pinnules, 

 seems to be to some extent dependent upon the condition of the respiratory apparatus 

 occupying their ventral surface. When this is well developed the arm seems to have the 

 power of indefinite growth. For in a great many individuals of various species which 

 have all the arms grooved and tentaculate like those of Antedon, there is no very 

 appreciable variation in their length or in the development of their genital glands. 



There appears to be no rule of any kind respecting the condition of the arms in any 

 given species of Actinometra. In the case of Actinometra parvicirra, for example, I have 

 seen individuals with thirty -three arms, all of which were grooved and tentaculate ; while 

 in another wdth thirty-one arms as many as nineteen were grooveless and unprovided 

 with tentacles. All sorts of gradations between these two extremes will be found in any 

 large collection of Actinometrce.' Half the species of this genus which were dredged by 

 the Challenger have more or fewer ungrooved and less developed arms. They may 

 occasionally be found upon the anterior rays ; while in Actinometra nohilis and 

 Actinometra magnijica,^ which have one hundred arms or more, several of those on each 

 ray are short and less developed, with neither food-groove nor tentacles on their ventral 

 surface (PL LVI. fig. 7). 



Even in the normal grooved arms of Actinometra the lower pinnules are frequently 

 grooveless and non-tentaculate, just as the hinder arms may be (PL LXI. fig. 3). Some- 

 times only three or four, sometimes as many as forty, are in this condition, being more 

 or less swollen by the development of the genital glands within them ; but they do not 

 receive any branches from the brachial ambulacrum, which is itself often but imperfectly 

 developed (see woodcut, fig. 4, p. 113). This ungrooved condition of the lower pinnules 

 may also occur on all the arms of some species of Antedon ; and it is especially remarkable 

 in types like Antedon accda and Antedon angusticalyx^ which have a strongly plated 

 ventral perisome. The ambulacral grooves of all the arms and of tke later pinnules are 

 well protected by plates (PL LIV. figs. 4, 7, 8, 9) ; but they do not extend on to (about) 

 the first twenty pinnules which contain the large genital glands, though the latter are 

 protected by a very close and regular pavement of anambulacral plates (PL LIV. figs. 

 1-3, 5). In other species, however, which have equally plated pinnules, such as Antedon 

 incefta,^ the ambulacra extend over their ventral surface in the usual way (PL LIV. fig. 6). 



1 Revision of the EcWni, part iv. pp. 680, 681. - Actinometra, loc. cil., pp. 31-41. 



3 The specific formula of this type is — a .3.2.3.3. -^ ■ 



* The following are the specific formula- of these types : A ntedon accela,~A . 10 . — ; ^ ntedon angusticalyx,—A . 3 . ^ . 

 5 The specific formula of Antedon incerta is — A. 10. — . 



(ZOOL. CHALL. EXP. — PART XSXII. — 1884.) K 8 



