146 THE VOYAGE OF H.M.S. CHALLENGER. 
The whole abactinal area is covered with large subcircular or irregularly oval plates, 
perforated throughout with circular holes; the drawing of the larval plates of another 
form on Pl. XXVI. fig. 6 indicates their character. The primary apical plates are large and 
very distinct; their relative proportions and arrangement will be better understood by 
a glance at the accompanying woodcut than by a lengthy verbal description. The sketch 
was made by myself with a camera lucida, and represents the exact form and position 
of the plates as seen under the microscope, without the slightest subsequent alteration 
or attempt at diagram-making. The large dorso-central (1) and the proximal cycle of five 
large contingent plates (2,2) are most conspicuous. The latter are radial in position, and 
appear to me to be the representatives of the under-basals of a Crinoid. If this view 
be correct their size and development is very remarkable. It also seems to be somewhat 
at variance with what is generally the order and proportion of development in other 
Asterids,j though hardly enough is known at present about the early stages of the group 
as a whole to warrant any dogmatic and positive statements to be formulated. External 
to the cycle of under-basals (2, 2) are five small circular plates (3, 3), interradial in position, 
and hence homologous with the basal plates. These plates have quite a different aspect 
from any of the others, and are much stronger and more compact in their structure. The 
radial plates are separated from the basals and from the under-basals by two or some- 
times three intermediate plates, and can scarcely be distinguished by their size from the 
general plating. The abactinal interradial areas are almost devoid of plates, only a 
few isolated and very small ones being present. They appear to be the plates which 
bear spinelets, and they show a tendency to become crowded towards the margin, near the 
cribriform organ. In the centre of the disk is a small space, enclosed by the dorso-central 
and four under-basal plates, occupied by membrane, and from this springs the small but 
elongate tubular anal funnel. 
The madrepodriform body is at the extreme margin, and is embraced as it were by 
the cribriform organ. There is one cribriform organ in each interbrachial are; these are 
already well developed, and have about two rows of modified spinelets on each side of 
the interradial sutural line, which will ultimately form “ lamelle.” 
There are four supero-marginal plates between the median interradial line and the 
terminal plate, the one next to the terminal being much smaller than the others, and 
all except the one next the terminal are longer than high. The infero-marginal plates 
are very low, but subequal in length to their companion supero-marginal plate; in com- 
parison with these, they appear mere band-like strips, and this circumstance might 
probably lead M. Perrier® to think that only one series of marginal plates was present in 
the examples he described under the name of Caulaster pedunculatus, if the same dispro- 
1 On the Homologies of the Primary Larval Plates of Brachiate Echinoderms, Quart. Journ. Micr. Sct., 
1884, n. s., vol. xxiv. p. 29. 
2 Comptes rendus (Dec. 1882), t. xcv. p. 1380. 
