338 COMPARATIVE ANATOMY CHAP. 



The shape and method of association of these bodies is of importance for 

 classification, especially for distinguishing one species from another. Nearly all 

 their various forms can be traced back, in a way which cannot here be further 

 described, to a common form, viz. to a very short rod, which tends to branch 

 dichotomously at each end. In some Dendrochirotcc (Psolus, Thcelia, etc.) the 

 calcareous bodies upon the (physiologically) dorsal side of the body attain a 

 specially large size (1 to 5 mm.), so that the back appears to the naked eye to be 

 covered with scales or plates (Fig. 228, p. 287). 



In the Dendrochirotcc an anterior part of the body, the proboscis, is invaginable. 

 At the posterior boundary of this proboscis (when cvaginated) five calcareous oral 

 valves are found in a few genera. When the proboscis is invaginated these come to 

 lie close together in the form of a rosette, which protects the aperture. In Psolus 

 these five oral valves are placed interradially, and each is a large triangular calcareous 

 plate (Fig. 228, p. 287) ; in Oolochirus, Actinocucumis, etc., they are arranged radially 

 and consist of compact masses of calcareous granules and ambulacral papill.-e. In 

 many Aspidochirota and Dendrochirota radially or interradially arranged anal 

 valves (anal plates or anal teeth) also occur round the anus. 



II. Echinoidea. 



The skeleton of the Echinoidea forms a plated covering called the 

 test, which encloses the viscera. The greater part of this test is 

 composed of the plates of the perisomatic system, since, as a rule, the 

 plates of the apical system (the central plate, the periproctal plates, 

 the basals and radials) occupy but a small, and even sometimes a 

 minute, area at the apical pole. There are, however, exceptions to 

 this rule, e.g. the Triassic genus Tiarechinus, in which a considerable 

 portion of the test is formed by the plates of the apical system (cf. 

 Fig. 231, p. 289). 



The form of the shell is thus, as a rule, in the Echinoidea, deter- 

 mined by the perisomatic skeleton. The horizontal outline of the 

 shell, i.e. the outline seen when an Echinoid shell is viewed from the 

 oral or the aboral pole, is called the ambitus. This ambitus in 

 regular Echinoids is, as a rule, strictly circular, or else pentagonal with 

 rounded corners ; less frequently it is oval, in which case the greatest 

 diameter of the ambitus need not coincide with the symmetrical axis. 

 In irregular Echinoidea the ambitus is symmetrical, being generally 

 elliptical (lengthened from before backward), or else egg- or heart- 

 shaped. 



In all Echinoidea, except the Spatangoida, the mouth lies at the 

 centre of the oral surface of the test ; in the Spatangoida it has shifted 

 anteriorly on this surface. The mouth, however, always remains the 

 centre round which the plates of the perisomatic skeleton are 

 grouped. 



We have already seen that in regular endocyclic forms, the anus 

 lies in the centre of the apical system, but in exocyclic forms 'it leaves 

 the apical system and enters the posterior interradius, where it may 

 approach the ambitus, or even cross it on to the oral surface, always, 

 however, remaining in the posterior interradius. 



