248 



PHYLUM ECHINODERMATA. 



T 



is usually absent in the adult. The axial organ and generative 

 rachis are not present. 



In the Holothurians or sea-cucumbers the body is elongated 

 in the direction of the long axis, at or near the two ends of which 

 are the mouth and anus respectively, and the radial water- vessels 

 are disposed in five equidistant meridional rows, extending from 

 the oral to the anal pole. The mouth is surrounded by a row 

 of tentacles and the animal lies with its long axis parallel to 



the substratum. Typically 

 the mouth and anus are 

 terminal, and there are five 

 usually double rows of tube- 

 feet which mark the radii 

 and pass from the oral to 

 the anal end of the body 

 (Fig. 177). In such cases 

 the symmetry is pentameral 

 and there is apparently no- 

 thing to mark one side of the 

 body from the other. But as 

 a matter of fact there are 

 two structures which are 

 not radially disposed. These 

 are the stone-canal and the 

 generative opening. The 

 generative opening, which is 

 single, is placed in the centre 

 of an interradius, usually 

 not far behind the tenta- 

 cular circlet, and the stone- 

 canal is in the middle line 

 of the same interradius. 

 This inter radius is called dorsal. It enables us to distin- 

 guish a ventral radius, a right and left ventral radius, and a 

 right and left dorsal radius. Our enumeration of the radii of 

 Holothurians (Fig. 182) is the same as that adopted for other 

 classes. It is based on the assumption that the madreporitic 

 interradius is the same in all cases. We need not repeat our 

 warning (p. 119) as to the inadvisability of basing important 

 speculations as to homologies on this assumption. It sometimes 



4f-J 



FIG. 177. Cucumaria with extended dentritically 

 branched tentacles T. Af tube-feet (from 

 Claus). 



