42 



THE ZOOLOGIST. 



edible — the Beches-de-mer, or TreiDangs, which are so abundant 

 in the Eastern Seas, and so much relished by the gourmands of 

 China. 



The possession of calcareous deposits is but one of the two 

 leading characteristics of the Eehinodermata : when we compare 

 a Starfish with a Crayfish we see that, while the latter has a 

 definite right and left side, or is bilaterally symmetrical, a Star- 

 fish has a number of arms or rays, or exhibits a radial symmetry : 



and in the next place we observe that, 

 though there are exceptions, such as 

 in the case of the common Sun- star 

 of our own shores, yet the ordinary 

 rule is that these rays are five in 

 number. Further, if we look to the 

 lower surface of a Starfish, we note 

 that along the rays there are arranged 

 double rows of little tubes or suckers, 

 and if we examine a Sea-urchin we see 

 that the rows of suckers are separated 

 by inter-radial spaces in which there 

 are no suckers. Now, if we examine 

 a typical Holothurian, such as a 

 Cucumaria (fig. 1) we find that there 

 are here five equidistant double rows 

 of suckers, which mark the five rays 

 and are separated from one another 

 by bare interradial spaces. 



In the possession, then, of a quin- 

 queradiate symmetry and of calca- 

 reous deposits in its integument a Holothurian conforms to our 

 idea of a typical Echinoderm. 



The mouth is placed at one end of the body and the vent at 

 the other ; round the mouth is a circlet of tentacles, which in 

 the simplest cases are ten in number — that is, there are five 

 pairs ; these tentacles may be withdrawn into the body, and in 

 some cases this retraction is aided by five radially placed muscles. 

 At the base of these tentacles is a circular vessel, containin"- a 

 fluid and giving off branches to the tentacles, and five larger 

 trunks which run along the rays of the body, and communicate 

 with the suckers ; this circular vessel, with its tentacular and 



Fig. 1. — A Holothurian, Cucum- 

 aria Inland, with the buccal 

 tentacles expanded. Twice the 

 natural size (Cassell's ' Natu- 

 ral History,' volri. p272). 



