216 COMPAEATIVE ANATOMY. 



Tlie function of tliese organs is not at all certain. The view 

 that they are respiratory organs is opposed to the fact that only one 

 of them has any connection with the network of blood-vessels, 

 while the other is merely attached to the body- wall, and projects into 

 the body-cavity. However, the fact that water is taken up by these 

 organs, and is again expelled, chiefly by the aid of the strong 

 muscular wall of the hind-gut, is of importance. 



In some Apodia (Molpadia borealis) they are only provided here 

 and there with branched caeca, while in others the number of caeca is 

 increased. Thus in M. chilensis not only is one of the trees divided, 

 but the rectum also bears a number of smaller trees. The organ 

 is divided five times in some Lisarmatidte. They are simpler in 

 chai'acter in Echinocucumis (E. typicus), where they form long fine 

 tubes, provided with one short branch only. 



The tree-like organs of the Holothurife are absent in the 

 Synaptfe, but there is an arrangement, which as yet is only veiy 

 incompletely understood; this consists of canals, which are placed 

 along the insertion of the mesentery and open into the coelom by 

 funnel-like ciliated orifices (Chirodota pellucida). 



Glandular organs are also present on the rectum of many 

 Holothuriee in addition to the tree-like organs. These — the Ouvierian 

 organs — either form unbranched cfecal tubes, which are inserted 

 singly or in thick tufts (Bohadschia, etc.), or they form racemose 

 organs (Molpadia), or, finally, filamentous canals, beset with lobate 

 tufts of glands, and arranged in a whorl (Pentacta and Muelleria). 

 They secrete a substance which forms fine sticky filaments, which 

 may serve as organs of defence. 



Ccelom. 



§ 175. 



The development of the coelom from a vesicular structure cut 

 off from the earliest rudiments of the enteron (§ 171) gives to this 

 cavity a different signification to that which it has in other divisions, 

 where the coelom is not formed from any part of the rudiments of 

 the enteron. The importance of this point must not be overlooked. 

 But it may well be supposed that the water-vascular system, which 

 is developed in just the same way, formed an apparatus which 

 primitively formed part of the coelom, and was connected with 

 the hind-gut. 



The two coelomatic tubes nipped off from the enteron gradually 

 increase in size, and by becoming attached in part to the enteron, 

 and in part to the body-wall, form the more or less spacious cavity 

 of the coelom. The mesenteric filaments or bands which pass 

 from the perisome to the enteron are to be regarded as the remains 

 of the walls of these primitive structures. 



As the radiate Echinoderm body becomes developed, the coelom 



