40 EMBRYOLOGY OF THE STARFISH. 



Formation of the Spines. — Such is the state of the abactinal system 

 when the pentagon of tentacles is composed of simple loops ; let us now 

 examine this system in more advanced larvte, at the time when the inner 

 fold of the loops has become triangular at the extremity. When seen 

 from the ventral side (PI. VII. Fhj. 8), we find that the small lobes have 

 become wart-like projections, surrounding the whole edge of the abactinal 

 system (PI. 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 profile, smaller tubercles are seen on each arm, identical, in every- 

 thing except size, with those of the edge ; the tubercles are young spines, 

 arranged in regular lines (PI. VI. Figs. 2, 4, 6) ; one row of four alternat- 

 ing on the edge of the abactinal system with one row of three, this again 

 with one of two, followed by single tubercles, 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 network 

 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 with that of the plates of the 

 abactinal surface, as shown in PI. VI. Fig. 10, /, l^, l^. The indentations 

 of the rays are now so well marked (PI. 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 network 

 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 de- 

 veloped (PL V. Figs. 9, 13, ;•') ; <i"<^ it is from this central cell that the 

 others diverge, growing smaller and smaller as they approach the edge. 



Tn the present stage of the young Starfish the anal extremity of the 



