viii ECHINODERMATA MORPHOLOGY OF SKELETON 341 



plane of symmetry can be recognised in the Holedypvida at the first glance, because 

 the anal area lias shifted out of the apical system, and into that interradius which is 

 called the posterior interradius. The same is the case in the Clypeastroida, and, in 

 a still higher degree, in the Spatangoida. In the Clypeastroida the peristome with 

 the mouth still remains in the centre of the oral surface, or only very slightly shifts 

 away from this position. But the ambitus is no longer circular or regularly 

 pentagonal ; its outline appears symmetrically lengthened or shortened in the 

 direction of the longitudinal axis, in such a way that, even in a superficial view, the 

 plane of symmetry is discoverable. Apart from the fact that the posterior inter- 

 radius is at once recognisable by the anus lying in it, it is often further distin- 

 guished in the Scittcllida'- by a perforation through the test (lunula), which never 

 occurs in the other interradii. Further, in the SciiMlido:, the bilateral symmetry 

 is often distinctly indicated by the number and arrangement of the radial lunulre, 

 or of the marginal incisions (Figs. 233-235, pp. 292, 293). 



The bilateral symmetry, which is most pronounced in the Spatangoida, culmi- 

 nates in the remarkable family of the Pourtalesiidce. The ambitus, which varies 

 greatly in details, is frequently egg-shaped, or heart-shaped, and in Pourtalesia 

 rtask-shaped. Not only does the anus always lie somewhere in the posterior inter- 

 radius, but the oral area also shifts from the centre of the oral surface, moving more 

 or less far along this surface anteriorly. In the Cassidulidce all the transition stages 

 between a central and a frontal position of the oral area occur. Since the mouth, 

 with the oral area, always forms morphologically the centre of all the systems of radii, 

 in shifting anteriorly it necessarily draws along with it the systems radiating out 

 from it. We shall return later on to the dissimilarity in the ambulacra, and especially 

 to the abnormal development of the anterior ambulacra, and consequent formation of 

 the bivium and trivium, to the special form of the peristome of the Spatangoida, etc. 



The apical system also does not always remain at the dorsal centre of the test, 

 but shifts more or less far forward (less frequently backward), and the highest point 

 of the test may then come to lie in front of (less frequently behind) its central 

 point (Figs. 236-238, pp. 294, 295). 



We have seen that in exocyclic Echinoidea (in which the anal area lies some- 

 where in the posterior interradius) the longitudinal axis and the plane of symmetry 

 can easily be made out even in a superficial examination, they can also be dis- 

 covered by careful observation, even in regular endocyclic Echinoidea, which are 

 apparently strictly radiate. When describing the apical system, the constant 

 relation of the outer apertures of the pores of the stone canal to the right anterior 

 basal plate, was pointed out. These relations never quite disappear, and where 

 the apical system is retained, they define with certainty the longitudinal axis 

 and the plane of symmetry. 



Further, even where the apical system has not been retained, it is always 

 possible, as has been proved by a very careful investigation of the Echiiioid test, 

 to determine the longitudinal axis and the plane of symmetry by the definite and 

 constant arrangement of the plates of the test, both in regular and irregular endo- 

 cyclic and exocyclic Echinoids. This constant relation of the plates to one another 

 is expressed in Loven's law. 



Let the test of any Spatangoid be laid with the dorsal (apical) side on a perpen- 

 dicular surface, in such a way that the mouth is directed upward, and the posterior 

 unpaired interradius (between the bivium) downward. Let the five ambulacra be 

 then marked with the figures I, II, III, IV, V (Fig. 302), starting from the left 

 lower ambulacrum (the right posterior of the animal) and proceeding in the direc- 

 tion in which the hands of a watch travel. Two plates of each ambulacrum, the 

 so-called marginal peristome plates, take part in forming the boundary of the 

 peristome. The first marginal plate which is met with in each ambulacrum, when 



