Chap. II. FORMATION OF THE SPINES. 33 



position of the water-pore (b) is immediately on the edge of the disk, at the extrem- 

 ity of the dorsal end of the pentagon (PL V. Figs. 10, 13, b). 



Formation of the Spines. — Such is the state of the abactinal system when the pen- 

 tagon of tentacles is composed of simple loops ; let us now examine this system in 

 more advanced larvae, at the time when the inner fold of the loops has become 

 triangular at the extremity. When seen from the ventral side (PL VII. Fig. 8), 

 we find that the small lobes have become wart-like projections, surrounding the 

 whole edge of the abactinal system (PL V. Fig. 9). These projections are composed 

 of accumulations of Y-shaped rods, connected with the system of network in the 

 larger plates. The surface of the abactinal system has also become covered with 

 these wart-like projections, rendering the outline irregular. In an abactinal pro- 

 file, smaller tubercles are seen on each arm, identical in everything except size, 

 with those of the edge; the tubercles are young spines, arranged in regular lines 

 (PL VI. Figs. 2, 4, 6) ; one row of four alternating on the edge of the abactinal 

 system with one row of three, this again with one of two, followed by single tuber- 

 cles, forming a pentagon, placed in the apex of adjoining rows, in the angle between 

 two arms; the older tubercles are those nearest the edge. 



When the young Starfish has reached this state, it has the rudiments of nearly 

 all the external parts of the adult. I shall, therefore, apply to these rudimentary 

 organs the names usually given to them. The spines are warts, not rising much 

 above the general level of the abactinal region, and they are arranged in regular rows. 

 The position of the net-work of limestone meshes has become well circumscribed, the 

 plates formed by them occupying the position of the original rods. The five smaller 

 plates in the angles of the arms are arranged round a central plate, the larger plates 

 alternate with them and occupy nearly the whole of the surface of the arm : this 

 arrangement is identical to that of the plates of the abactinal surface, as shown in 

 PL VI. Fig. 10, /, / 1; 4- The indentations of the rays are now so well marked (PL V. 

 'Figs. 12, 13) that there is quite a large open space between the outer spines on the 

 edge of any two adjoining arms. On examining the plates formed by the net-work 

 of limestone meshes, we see that the cells are polygonal ; they are usually hexagonal, 

 and are more or less quadrangular near the exterior of the plate. The original rod 

 can be recognized by the larger cell it has developed (PL V. Figs. 9, 13, r) ; and it 

 is from this central cell that the others diverge, growing smaller and smaller as 

 they approach the edge. 



In the present stage of the young Starfish, the anal extremity of the Brachiolaria 

 (PL VII. Fig. 8) has almost entirely disappeared, and the embryo Starfish has taken 

 its place (PL V. Figs. 9-14). This embryo is so heavy that, when floating about, 

 it loads down the anal part, which is always the lowest, and the larva is com- 

 pelled to move always more or less obliquely, having to drag this great weight 



vol. v. 5 



