454 ECHINODERMATA ASTEROIDEA chap. 



In the family Heliasteridae the mutual adhesion between the 

 arms has gone on merely to a slight extent, for the interradial 

 septa are still double. 



Skeleton. Most of the schemes of classification have been 

 founded on the skeleton, largely because the greater number of 

 species have only been examined in the dried condition, and 

 little is known of their internal anatomy or habits. There is, 

 however, this justification for this procedure, that the habits and 

 food of the species (with the exception of the Paxillosa) which 

 have been observed in the living condition appear to be very 

 uniform, and that it is with regard to the skeleton that Asteroidea 

 seem to have split into divergent groups through adopting 

 different means of protecting themselves from their foes. 



The description of the various elements of the skeleton will 

 be arranged under the following heads : (a) Main framework ; 

 (b) Spines ; (c) Pedicellariae ; (d) Ambulacral skeleton. 



(a) Main Framework. The type of skeleton which supports 

 the body-wall of Asterias is called reticulate. As already indi- 

 cated it consists of a series of rods bound together by bundles of 

 connective-tissue fibres so as to form a mesh-work. This is a very 

 common type of aboral skeleton, but in a large number of Starfish 

 a different type occurs, consisting of a series of plates which may 

 fit edge to edge, leaving between them only narrow interstices, as 

 in the Zoroasteridae, or which may be placed obliquely (as in 

 Asterina) so that they imbricate or overlap one another. In a very 

 large number of Asteroidea the supero- and infero-marginal ossicles 

 are represented by squarish plates even when the rest of the 

 skeleton is reticulate ; this is the so-called " phanerozonate " 

 structure, the term " cryptozonate " being used when the marginals 

 are rod- like and inconspicuous. In other cases (Ganeriidae) the 

 whole skeleton of the ventral surface is made of tightly fitting 

 plates, whilst the aboral skeleton is either reticulate or made of 

 imbricating plates. Lastly, the skeleton may be represented only 

 by nodules forming the bases of paxillae (see p. 455), as in the 

 Astropectinidae, or may be entirely absent over wide areas 

 (Brisingidae). 



(h) Spines. The spines vary more than any other part of 

 the skeleton. They may be close set and small, or few and 

 large, and often bear spines of the second order, or spinelets, 

 attached to them. In Asterias and its allies they are com- 



I 



