DEATilLOPMENT OF ANTEDON (COMATULA, LA:\rK.) "ROSACEUS. 713 



which this pentagonal base is composed, as to seem in absolute continuity with them 

 (Plate XXXV. tig. 1,/); whilst its central part also is frequently connected with it 

 by delicate processes, which sometimes sprout forth irregularly from the inner margins 

 of the component pieces of tlie pentagonal disk, but sometimes form a more regular 

 ingrowth, which considerably contracts the central space on the ventral aspect of the 

 disk (Plate XXXIII. figs. 1, 3), and becomes continuous with an annular projection 

 from the ventral face of the rosette. 



32. The Pentagonal Base may be readily separated, after continuous boiling in the 

 potass-solution, into its component First Eadials, their mutual adhesion, and their 

 adhesion to the centro-dorsal plate, being due to the interposition of a thin layer of sar- 

 codic substaiice, continuous AAith that which occupies the meshwork of their own calca- 

 reous reticulation. The extreme fragility of the delicate processes whereby they are 

 severally connected with the " rosette," occasions their detachment from it when they are 

 separated from one another. In the adult condition of A. rosaceus, the " rosette" itself is 

 not resolved into distinct pieces by any amount of boiling ; although in its immature 

 stage it is readily separable into five component plates (§ 90). — The pieces of which the 

 Pentagonal Base is made up will now be described in detail. 



33. First Radials. — Each of these pieces has a well-marked triangular form, the 

 apical portion of the triangle, however, being deficient (Plate XXXVI. fig. 1, c, d). 

 AVe may distinguish its ventral and its dorsal faces (c, d), the former looking towards 

 the concavity of the calyx, whilst the latter is in contact with the centro-dorsal plate ; 

 its internal face (b) bordermg the central space of the pentagonal disk; its two lateral 

 surfaces (b, /, i) by which it adheres to its fellows; and its external face (a), by 

 which it is articulated with the Second Kadial. The ventral face (c) is divided by two 

 curved ridges (fZ, f/), bending towards each other along the median line, and there sepa- 

 rated by a furrow, into a central and a peripheral portion, the former sloping inwards, 

 so as to contribute to the formation of the central funnel ; whilst the latter slopes 

 outwards, so as in fact to become part of the external face of the plate. The dorsal 

 face (d) is slightly convex, but is free from irregularity, except that there is a deep 

 notch (li) in the centre of its inner margin. The two lateral faces (b, i, i) are perfectly 

 flat ; and their only feature is a large aperture {g, g) which each presents towards its 

 internal margin. The internal face (b), which is comparatively small and irregular, shows 

 near its dorsal margin a pau* of large apertures (e, e), the edges of which are raised 

 so as to leave a distinct fuiTow (/;) between them ; this furrow, which has shown itself as 

 a notch in the inner margin of the dorsal face (d, h) is continued onwards towards the 

 ventral margin, but is more or less interrupted by the irregular processes which extend 

 themselves to meet the rosette [^ 31), and which not unfrequently complete these furrows 

 into canals (Plate XXXIII. fig. 1). — The external face is divided by a strong transverse 

 ridge (a, a, a) into an upper and a lower portion ; and the upper is again di^ided by a 

 less elevated transverse ridge, and by median continuations of the ridges already noticed 

 .on the upper surface, into two pairs of fossa?, b, b, and c, c. Of these the upper pair 



5e2 



