282 
BLADEN : STRUCTURE OF ASTEROIDEA. 
general character of ophiuran structure, arranged and modified 
according to the plan of astsroid organization, and in this manner 
to reflect as it were, a primitive stage of the race-history of the 
classes which we are discussing. There exists, however in the 
anatomy of Astrophiura — for such is the name under which I have 
described* this aberrant genus — a feature that bears so strikingly 
upon the subject with which we are dealing, that it will be found 
not unworthy of attention whilst we touch upon it very briefly. 
The diagram given in Fig. 3, represents a section through one 
of the radii. On comparing this with those which we have previously 
been considering, it will be seen we have here a sort of compromise 
between the characters of both. In a modified form there is the 
internal skeleton belonging to the Ophiuroidea, together with the 
presence of the peritoneal cavity which is found in Asteroidea. The 
radial axis however is extremely aborted, and the disk-like process- 
es, which in the Ophiuroidea extend from side to side across the 
section of the ray, are here reduced to diminutive ear-shaped 
processes of the most insignificant description. (Fig. 3, a.) 
Furthermore, there exists a supplementary plate in Astrophiura 
(Fig. 3, e.), and upon this especial interest devolves. It consists 
of a large, broad, thin plate, that joins up to the aborted disk-pro- 
cesses of the axis and forms a partition reaching up to the inner 
surface of the abactinal wall of the test, constituting in fact the 
divisional septum, or wall of a compartment, for an ambulacral 
tentacle. No equivalent for this accessory plate can be pointed 
to in the section of the typical Ophiuroid, and search has to be 
made amongst the Asteroide*a before any homologous structure is 
met with. In the more ancient forms of that group, such e.g. as 
the Astropectinidce and Linckiadce, a small supplementary plate 
occurs, which forms a connective or intermediate piece, filling in 
the angle formed by the ambulacral and ventro-lateral plates. (Fig. 
1, e.) These are what I consider to be the homologues of the sup- 
plementary plates or ambulacral septa just mentioned in Astro- 
* Proceed. Roy. Soc, vol. xxvii, p. 456. Ann. and Mag. Nat. Hist., ser.5 
vol. iv, p. 401, pi. xx. 
