TYPE ECHINODERMA. 545 



plates disappear during growth, the disk being either naked, 

 or covered by a number of small plates which are termed an- 

 ambulacrals, certain of which lying on either side of the ambu- 

 lacral grooves receive the special name of ddambulacrals, or 

 covering-plates. 



The ectoderm cannot usually be distinguished over the 

 surface of the calyx or on the stem, but is present on the 

 disk and on the oral surfaces of the arms and pinnules, being 

 there non-ciliated except along the ambulacral grooves. It 

 rests upon a connective tissue in which the calcareous plates 

 are developed, and from which strands, frequently with cal- 

 careous spicules imbedded in them, usually traverse the body- 

 cavity. The ligaments which unite the plates of the arms 

 and stem are formed of this connective tissue, and contractile 

 fibres of a peculiar character are sparingly developed in it, 

 stretching across the non-syzygial joints of the arms, pinnules, 

 and cirri, and probably also reaching a slight development 

 in the stern. 



The internal structure of the Criuoids is knoAvn princi- 

 pally from observations on Antedon, and the following account 

 represents what occurs in that form. The ccelom, as already 

 stated, is traversed by numerous strands of connective tissue, 

 and primarily consists of two cavities separated from each 

 other by a mesentery, each cavity being continued out into 

 the arms, forming the oral and aboral canals of these struc- 

 tures, at the extremities of which they unite. The mesentery 

 does not, however, long persist in its entirety, but the two 

 cavities fuse, new membranes, however, arising and dividing 

 them in some species of Antedon. One of these membranes 

 (Fig. 251, vs) surrounds the intestine and forms the visceral 

 sac, its presence rendering the evisceration of Antedon an 

 easily-accomplished process and one which is made use of 

 by the animal in unfavorable conditions, a new visceral mass 

 being later regenerated. The portion of the ccelom which 

 lies peripherally to this sac is termed the circumvisceral 

 portion (cc), and that within it the intervisceral (ic), the latter 

 containing an axial cavity (Axi) enclosed by a membrane sim- 

 ilar to the visceral sac and continuous with the oral coelomic 

 cavities (oc) of the arms, the aboral cavities (ac) communicat- 



