354 COMPARATIVE ANATOMY 



(c) The Accessory Skeletal System. 



In this system may be included all those plates or ossicles which occur in those parts 

 of the body not covered by the ambulacral and marginal systems. This accessory system 

 is very variously developed, and a comparative study of it cannot here be under- 

 taken. The plates differ greatly in size, form and ornamentation, and arrangement, 

 sometimes being scattered or lying loosely near one another, or else closely approxi- 

 mated, sometimes imbricating or reticulating by means of anastomoses of skeletal 

 pieces. 



Not infrequently either theAvhole, or parts, of the accessory skeleton are reduced. 

 It is often covered by a consideraljle layer of integument, and is difficult to dis- 

 tinguish externally. Its plates may diminish greatly in size, even becoming micro- 

 sco])ically small, but they are rarely altogether wanting. 



Three sub-divisions of the accessory skeleton may be distinguished : — 



1. The dorsal, abactinal, or apical accessory system, when present, consists 

 of skeletal plates developed in the dorsal integument of the disc and in the arms. 

 We have seen above that in the Asteroidea the apical system only rarely takes any 

 recognisable part in the formation of the dorsal skeleton. There are, nevertheless, 

 forms {c.ff. Cnemidastcr) in which the large and distinct plates of the apical system 

 form almost the whole of the dorsal protection of the disc. 



2. The ambital accessory system consists of the intermarginal plates already 

 mentioned as occasionally being intercalated between the supra- and the infra- 

 marginal rows of plates. 



3. The ventral, actinal, or oral system in the same way consists of the already 

 mentioned intermediate plates which may occur between the inframarginal and the 

 adambulacral plates. It is most developed in those forms in which the disc 

 increases in size at the expense of the arms, i.e. in forms whose outline is more or less 

 pentagonal. The ventral accessory plates then fill up the larger or smaller triangular 

 regions between the ambulacral furrows on the lower side of the disc. 



Finally, two other skeletal systems which occasionally occur in the Asteroid 

 body must be mentioned. 



In a certain number of Star-fish each ambulacral ossicle is connected by a skeletal 

 plate, or more rarely by a row of two to three firmly united plates, through the 

 body cavity, with a marginal plate of its own side, or else with a laterally placed 

 accessory plate. These simple or compound skeletal pieces, which are limited to 

 the arms, and which here correspond in number with the ambulacral ossicles, are 

 called supports to the ambulacral ossicles or supraambulacral plates (Fig. 309 sa). 



The other skeletal system, which occurs especially in Asteroidea with large discs, 

 but is altogether wanting in many forms, is called the interbrachial system. It 

 continues the divisions between the arms, either completely or incompletely, into 

 the interior of the disc, and consists either of interbrachial walls, running from the 

 oral to the actinal skeleton, or of interbrachial chains of skeletal plates descending 

 vertically to the oral skeleton. In each interradius a proximal plate of this inter- 

 l)rachial skeleton, however, always enters into closer relations with the oral skeleton. 

 These plates are the orals, already mentioned in the section on the oral system. 



At the free end of each arm in every Asteroid there is to be 

 found a single median skeletal plate, which is sometimes of consider- 

 able size and distinctly visible, sometimes small and inconspicuous ; 

 it carries on its lower side a visual organ. These plates are called 

 ocular plates or terminals. According to recent investigations they 

 develop very early (apparently first of all the plates) over the left 



