432 COMPARATIVE ANATOMY CHAP. 



No very sharp distinction between tube-feet and papillae is, however, possible, 

 either with regard to their distribution, their form, or their structure. 



With regard to the number of the tentacles, the following numbers seem to 

 prevail in the different families : 20 in the Aspidochirotce, 20 in the sub-family 

 Dcimatidcc of the Elasipoda, 15 in the Molpadiidcc, 13-16 in the Pelagothurida; 12 

 in the Synaptidce, and 10 in the Dcmlrochirotcc and in the sub-family Elpidiidcc 

 of the Elasiptxlit. 



With regard to form : the tentacles are feathered (Molpadiidcc Synaptidce, Fig. 

 229, p. 288), dendriform (Dcndrochirotce, Fig. 226, p. 287), and shield - shaped 

 (Ax/>idochirotcc, Elasipoda). In the latter, the disc or shield, the edge of which may 

 be more or less deeply indented, is carried by a stalk. 



The size of the tentacles has already been sufficiently indicated in the systematic 

 review. 



The relation between the arrangement and size of the tentacles on the one hand 

 and the symmetry of the rest of the body on the other is interesting. In the 

 Dcndrochirotce (cf. Fig. 226, p. 287), of the ten tentacles, the two ventral are almost 

 always distinguished by being much smaller than the rest. 



In many species of Myridroektu, Synapta, and Chirodota with twelve tentacles, 

 these are distributed symmetrically as follows : three occur in each of the two 

 dorsal interradii, and two in each of the three ventral interradii. 



The tentacles may be swelled and extended ; and on the other hand they can be 

 withdrawn into the body cavity together with the surrounding anterior part of 

 the body, although not iuvaginated like the tentacles of a Gastropod. 



2. Echinoidea. Ambulacral feet are developed in all Echinoids 

 without exception. In early youth, they are always found to resemble 

 one another, and in both the EcMnidce and the Pourtalesiidce this 

 is still the case in adults, the former having tube-feet with terminal 

 sucking discs and the latter tube-feet with rounded ends. In most 

 Echinoidea, on the contrary, more or less marked polymorphism 

 occurs, division of labour taking place between the ambulacral 

 appendages of one and the same individual. 



This polymorphism is not very striking in the regular Echinoidea, 

 e.g. the Cidaroida, Echinothuridce, Diadematidce, Arbaciidce, Echinometr'nlti , 

 etc. In these, the ambulacral appendages appear, as a rule, in three 

 different forms : (1) as loeomotory tube-feet with terminal or sucking 

 discs; (2) as tactile or branchial tentacles without terminal sucker; 

 and (3) as oral or sensory feet with bi-lobate terminal disc. 



All these tube-feet are connected by means of double pores with their ampullae, 

 which lie within the test. Without detriment to their principal function, they 

 may all act as respiratory organs, since the presence of the double pore allows of 

 a circulation of the ambulacral fluid between the inner ampulla and the outer 

 ambulacral foot ; the fluid in the foot takes in oxygen, carries it back into the 

 ampulla, and gives it off through the wall of the ampulla to the fluid in the body 

 cavity. 



The loeomotory tube-feet are found on the oral hemisphere of the body, but 

 may also occasionally occur on the apical hemisphere as well. 



The tactile or branchial tentacles are limited to the apical hemisphere. They 

 are specially suited for respiratory purposes when the ampullae are large, and have 

 thin and delicate walls containing no calcareous corpuscles. 



