ECHINODERMATA 



IOI 



Many of the Holothuroidea, in fact, resemble the worms much 

 more closely than they resemble the other Echinodermata ; for 

 in this type the chief axis of the body, which extends from the 

 oral to the aboral pole, is relatively short as a rule, but in 

 the Holothuroidea it is greatly elongated, the mouth being at 

 one end and the anus at the other. The body wall is almost 

 always soft, more or less leather}', and apparently destitute of 

 a skeleton ; but calcareous 

 plates are often present in 

 the skin, and these plates are 

 often elaborate and ornamen- 

 tal structures, taking the form 

 oi rosettes, anchors, sieve 

 plates, etc. The mouth is 

 surrounded by a circle of soft 

 tentacles, varying in number 

 from ten to thirty, sometimes 

 short, often long and beauti- 

 fully branched (Fig. 93). The 

 surface of the body may be 

 perfectly smooth, but gener- 

 ally it is marked by five longi- 

 tudinal bands, corresponding 

 to the ambulacral areas on 

 other Echinodermata, and the 

 spaces between them to the 

 interambulacral areas (Fig. 

 93). On these bands there 

 are sometimes ambulacral 

 feet, more often merely tenta- 

 cles, and in some species there are tentacles attached irregularly 

 over the whole body. The radial symmetry indicated by the 

 ambulacral and interambulacral areas, however, is generally 

 masked by the bilateral symmetry which these animals have 

 acquired by lying habitually on one side, represented by two 

 interambulacral areas and the adjacent three ambulacral areas, 

 which are called the trivium, the remaining two the bivium. 

 Thus the sea cucumbers have as a rule pretty distinct upper 

 and lower surfaces, which are further more or less modified in 



FIG. 93. Cucumaria plana. Ventral aspect, 

 with tentacles expanded and tube feet 

 extended. (After Ludvig, from Parker and 

 Haswell's Manual.) 



