406 COMPARATIVE ANATOMY CHAP. 



in reality only characterised according to their position in the adult animal, whether 

 radial or interradial, apical or oral, and according to the place of their first appear- 

 ance (above one or the other ccelomic vesicle). They have no other distinctive 

 characteristic by which, for example, a radial could be recognised throughout the 

 class of the Echinodermata. It is therefore still possible that such correspondence 

 may be merely superficial, merely the expression of the radiate structure so common 

 among Echinoderms. There is nothing astonishing in the fact that the skeleton 

 of a radiate animal commences at the poles either with radially or with interradially 

 arranged plates. Such correspondence, then, as far as it goes, is described as homo- 

 logy. But nothing is really gained by insisting that such and such Ophiurids 

 " possess infrabasals," because the system of plates commences at the apex with five 

 radial plates, which are followed by another outer row of radial plates. Is it, after 

 all, certain that the infrabasals are wanting when the skeletal system at the apex 

 begins with interradial plates (which are on that account called basals) ? 



III. The Outer Morphology of the Holothurioidea. 



The Holothurioidea form an exception to the rule which applies to all other 

 Echinoderms that the outer form of the body is accurately reproduced in the test of 

 skeletal plates. This test gives us, as a rule, exact information as to the position 

 of the outer apertures of the internal organs, and as to the relation of the radii 

 or ambulacra, to the interradii or interambulacra. But in the Holothurioidea, in 

 whose integument only microscopically small and isolated calcareous bodies occur, 

 this is not the case. Having treated of the external morphology of the Echinoidea, the 

 Asteroidea, the Ophiuroidea, and the Pelmatozoa in the section on the skeletal system, 

 we must now give some account of the outer morphology of the Holothurioidea. 



We shall begin with those forms in which the body, elongated in the 

 direction of the principal axis, is, in section, circular or pentagonal with 

 rounded edges (cf. for example, Cucumaria Planci, Fig. 226, p. 287). At 

 the oral pole of the principal axis (i.e. in the Holothurioidea, anteriorly) 

 lies the mouth, surrounded by feelers ; at the opposite, apical (posterior) 

 pole, the anus. Along the body, from before backward, run five ridges, 

 corresponding with the radii, and causing the pentagonal form of the 

 transverse section. On each edge there are two longitudinal rows of 

 tube-feet. 



Careful examination shows that the radiate structure of Cucumaria 

 is even externally disturbed by certain characters which make it bi- 

 laterally symmetrical. There is only one genital aperture, at the oral 

 margin of an interradius, which we will arbitrarily call the dorsal 

 interradius. Further, of the ten oral feelers, two adjacent feelers 

 are much smaller than the rest. They lie exactly opposite the genital 

 aperture, and distinguish the middle ventral radius. The plane which 

 passes through the dorsal interradius and the middle ventral radius, in 

 the direction of the principal axis (i.e. longitudinally through the body) 

 is the plane of symmetry. 



If the animal is opened, it is seen that this external symmetry 



corresponds with an internal symmetry ; the anterior limb of the 



A_intestme is attached by a mesentery to the body wall in the dorsal 



X ! The stone canal and the genital glands lie in the dorsal 



interradius, and the Polian vesicle in the middle ventral radius. 



