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 PI. 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, 1 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 arc ; 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 "lamellae." 



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 2 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. Sci., 

 1884, n. s., vol. xxiv. p. 29. I 



2 Coviptes rendus (Dec. 1882), t. scv. p. 1380. 



