280 THE VOYAGE OF H.M.S. CHALLENGER. 



of the Echinoderms secrete calcareous matters which take a distinct form, and that 

 the perisome especially possesses this secreting function in a very high degree. Even 

 in the very early stages of development, the Echinoderms are characterised by pos- 

 sessing calcareous deposits, which almost always take the form of simple or branched 

 spicules, which, as the larvae grow larger, change their form and become more or less 

 complicated. Thus it is a fact that many of the fully- developed shallow water Holothu- 

 rians have their body- wall supported by plates, crowns, anchors &c, and that their 

 calcareous ring, which is often very firmly constructed, is made up of a solid calcareous 

 network. It is of importance to note that the Elasipoda, with few exceptions, present a 

 singular resemblance to the larval forms as to their calcareous deposits in the perisome 

 as well as in the ring surrounding the gullet, these deposits having been arrested at a 

 very low degree of development. 



" The water- vascular system is very well developed in the Elasipoda, and has some- 

 times a more complicated conformation than is met with in the pedate shallow water 

 Holothurians, their ampullae, which are sometimes branched, having attained a very 

 considerable size and being enclosed within the perisome. All the manuals of inverte- 

 brate zoology indicate, as an important character distinguishing the Holothurians from 

 the rest of the Echinoderms, that the water- vascular system in the former communicates 

 with the interior of the peritoneal cavity by the madreporic canal, whde in other 

 Echinoderms the same canal opens into an exterior madreporic tubercle, thus placing 

 the ambulacral system in direct communication with the surrounding medium. This 

 character must now be omitted, since it has been observed that many of the Elasipoda 

 are remarkable in having the water- vascular system in persistent communication with 

 the exterior, and that too not only by one pore but sometimes by a great number of 

 pores crowded closely together, so as to form a kind of external madreporic tubercle^ 

 The larvae of the Apoda and Pedata have the madreporic canal in communication with 

 the surrounding medium by an opening on the dorsal surface ; but eventually this canal 

 looses its connection with the exterior, so that it hangs loosely in the peritoneal cavity 

 of the adult animal. Thus the Elasipoda, even in this respect, obviously resemble the 

 larval state of other Echinoderms. 



" The respiratory trees are present in all the Pedata and in some of the Apoda, but in 

 most of the latter the respiratory trees are supplied with " ciliated cups." The true 

 function of these peculiar organs is unknown, though it seems probable that they 

 subserve an excretory function. The Elasipoda seem to be devoid of every trace of 

 these organs. Many authors seem to attach so much importance to the respiratory trees, 

 that two orders have been founded, Pneumonophora and Apneumona, on the presence 

 or absence of these organs. For my own part, I think that the presence or absence of 

 ambulacral appendages — that is to say, a more or less complicated ambulacral system — 

 is of far greater systematic importance, considering that the water-vascular system is one 



