436 ECHINODERMATA — ASTEROIDEA chap. 



separated from its fellow, remaining, however, connected with 

 the latter by a powerful adductor muscle. In consequence of the 

 separation of this pair of ossicles each is brought into closer 

 contact with the corresponding ossicle in the adjacent radius, to 

 which it is connected by a muscle called the abductor. The 

 first adambulacrals in adjacent radii are also brought into closer 

 contact and carry long spines which, when the ambulacral 

 grooves are contracted, project like a grating over the mouth. 

 In the order of Asteroidea to which Asterias belongs, the adam- 

 bulacrals themselves do not project much, but in all other cases 

 they form prominent mouth-angles, so that the opening of the 

 mouth becomes star-shaped (Fig. 211, p. 483). 



Except in the case of the ambulacral and adambulacral plates 

 little regular arrangement is to be detected in the ossicles of the 

 skeleton which, as has already been mentioned, form a mesh- 

 work. If, however, the arm be cut open and viewed from the 

 inside it will be seen that the edge is strengthened above and 

 below by very thick, powerful, rod-like plates. These are called 

 the " supero-marginal " and " infero-marginal " ossicles ; they are 

 not visible from the outside, since they are covered by a thick 

 layer of the body- wall containing other smaller plates (Fig. 190, 

 marg). In many genera, however, they are exposed, and form a 

 conspicuous edging to the arm above and below. In many 

 genera, also, there are three conspicuous series of plates on the 

 back of each arm, viz. a median row, called " carinals " {car., 

 Fig. 191), and two lateral rows, termed " dorso-laterals " {d.lat., 

 Fig. 191). These three rows, with the two rows of marginals, 

 one of ambulacrals, and one of adambulacrals on each side (11 

 rows in all), constitute the primitive skeleton of the arm, and 

 appear first in development. 



The structure of all these elements of the skeleton is the 

 same. They may be described as scaffoldings of carbonate of 

 lime, interpenetrated by a mesh-work of cells fused with one 

 another, by which the carbonate of lime has been deposited. The 

 matrix in which the ossicles lie is a jelly-like substance traversed 

 by a few bands of fibres which connect the various rods with one 

 another. This jelly is almost fluid in the fresh state, but when 

 heated forms a hard compound, possibly allied to mucin, which 

 will turn the edge of a razor. 



When the covering of the back is dissected off the coelom is 



