200 COMPAEATIVE ANATOMY. 



are intermediate towards those forms wliicli have lost their locomotor 

 function, and have the form of ambulacral branchiae or tentacles 

 (tentacular organs). 



When they are filled with fluid the feet swells and are thereby- 

 erected and capable of protrusion. The extent to which they are 

 protruded depends on the length of the firm appendages of the 

 integument, so that the sucker feet are longest in the Echinoida with 

 their long spicules. The end becomes fixed when the sucker is pro- 

 truded, so that the foot is now able, when it contracts, to draw the 

 body of the animal towards the point of attachment ; this mode of 

 locomotion is often eifected with great rapidity by the Echinoida. A 

 whole group of suckers takes part in the movement, and by working 

 together display considerable energy. The distribution of these 

 structures has been referred to in the previous paragraphs ; their 

 relation to the vascular system will be spoken of when that system 

 is dealt with. 



In the Crinoida the circumoral suckers acquire the function of 

 tentacles ; this is in many other cases combined with the locomotor 

 functions. But we get also independent tentacular structures, 

 allied in origin to these, namely, the tentacles in the neighbourhood 

 of the oral aperture of Holothuroida (cf. Fig. 113, T). They are 

 sometimes pinnate, sometimes branched, and, as a rule, can be 

 completely drawn in. In many Synaptte they carry suckers (S. 

 Duverntea). Their internal cavity is in communication with the 

 same vascular system, as are the ambulacral feet. 



The so-called dermal branchiae, tentacles which are dis- 

 tributed over the antambulacral (dorsal) surface of the body in 

 the Starfishes (Fig. 100, t), differ from these; in the Echinoida 

 they form five pairs of contractile tree-like organs near the mouth. 

 They communicate with the body-cavity, and are merely protrusions 

 of the integument. 



Integument and Dermal Skeleton. 

 § 163. 



In the Echinoderma the same dermo-muscular layer is present as 

 in the Vermes, but the integument is more sharply marked off from 

 the musculature. This latter forms, for the most part, a layer which 

 limits the body-cavity, and is covered externally by the integument. 

 Its special characters are due to the fact that the locomotor power of 

 the body is more or less determined by the deposition of chalk 

 in that layer, which goes to form, in conjunction with the muscular 

 layer, the " perisome." 



The same structure exists in the larva; but in it this exoskeleton 

 is never of any great size, but is merely a firm support formed by 

 a large number of rod-like processes. The ciliated bands extend on 

 to the processes, which theyfringe, and form, in a more or less com- 

 plicated manner, the locomotor apparatus of the larva (vide Fig. 95, 



