VIII ECHINODERMATA— SPINES, ETC. 387 



In an interradius on the oral side of tlie body, nearer one of the ambulacra tlian 

 the other, lies the anus, whicli can be closed by valves. 



Mesites shows a certain similarity with the Palaeeehinoidea. By arbitrarily 

 assuming that an anibulacral vessel ran in the canal below the ambulacral plates, 

 and that anil)ulacral feet passed out through the pores between these plates, stress 

 was laid upon the agreement thereby established between the Echinoids and Mesites 

 in the position of the radial water vascular trunk on the inner side of the ambu- 

 lacral plates. But (1) it is quite uncertain that the ambulacral vessel really lay 

 in this canal and not in the outer channel, (2) the ambulacral feet, in Echinoids, 

 pass through the ambulacral plates and not between them as in Mesites, and (3) it is 

 not at all certain' that tlie pores of Mesites really served for the passage of ambu- 

 lacral feet. 



Agelacrinus (Fig. 262, p. 313). — The body resembles a more or less flat, round 

 disc, attached to a firm object {e.g. the shell of a Brachiopod). The test is formed 

 of numerous, irregularly arranged scale-like plates, which more or less imbricate 

 with one another. In the middle of the free (oral) side of the disc lies the mouth, 

 covered with plates, from which five curved ambulacral furrows radiate, covered 

 with double rows of alternating plates. The plates of each double row form a 

 tunnel, raised above the level of the disc, and, between the plates, apertures may 

 occasionally be observed which are supposed to have served for the passage of 

 ambulacral feet. In one of the interradii, between two ambulacra which converge 

 so as to form a ring, lies the anus, arched over by a pyi-amid of valves. 



Just as Mesites has been regarded as nearly related to the racial form of the 

 Eeliinoidea, so Agelacrinus (with the related genus Edrioaster) was said to be nearly 

 connected with that of the Asteroidea. But it appears hardly conceivable that the 

 almost rigid skeleton of the attached, sessile Agelacrinus could give rise to the 

 richly jointed and movable skeleton of the Asteroid. In the Asteroids, it is not 

 the ambulacral feet but the canals connecting them with the ampullie which jjass 

 out between the ambulacral plates, and the radial water vascular trunks lie outside 

 of the latter. The double rows of covering plates in Agelacrinus cannot thus be 

 compared with the double rows of ambulacral plates in the Asteroid. 



In conclusion, it may here be remarked that structures resembling the 

 pectinated rhombs of the Cystidea occur in many fossil Crinoids and Echinoids. 

 These are parallel strire on the skeletal plates, which run transversely over the 

 suture dividing two neighbouring plates, and together form a rhomb -like figure. 

 In young and fossil Echinoids, it is principally the plates of the apical system 

 which are ornamented with such striated rhombs. 



D. The Spines and their Derivatives — The Sphseridia and the 

 Pedieellariae. 



1. The Spines. 



The test of the Eeliinoidea, and the plated test of the Asteroidea and 

 Opliiuroidea carry large or small, variously shaped spines or processes, 

 which also vary in number and arrangement. The knowledge of 

 the structure, shape, size, and arrangement of these rigid processes 

 of the body (aeanthology) is of importance for classification. We 

 must here confine ourselves to the most important points, referring 

 the reader for further particulars to systematic works. 



a. The Spines of the Eehinoidea, which we shall first consider 



