i 
REPORT ON THE CRINOIDEA. 57 
A similar inequality in the development of the genital glands has been noticed by Prof. 
A. Agassiz as occurring 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 m 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 with 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 Actinometre.’ 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 nobilis and 
Actinometra magnifica,’ 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 
erooveless 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, bemg 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 acala and Antedon angusticalyx,* which have a strongly plated 
ventral perisome. The ambulacral grooves of all the arms and of the 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 (PI. LIV. figs. 
1-8, 5). In other species, however, which have equally plated pinnules, such as Antedon 
incerta, the ambulacra extend over their ventral surface in the usual way (Pl. LIV. fig. 6). 
1 Revision of the Echini, part iv. pp. 680, 681. 2 Actinometra, loc. cit., pp. 31-41. 
3 The specific formula of this type is—a.3.2.3.3. = : 
4 The following are the specific formule of these types: Antedon acela,—A .10. a Antedon angusticalyx,—A . 3. - : 
® The specific formula of Antedon incerta is—A . 10. < ; 
hs 
(ZOOL. CHALL, EXP,—PART xxxI.—1884.) 
