VIII EGHINODERMATA— MORPHOLOGY OF SKELETON 349 



still not insignificant plates (episternum). The test is then amphisternal. In 

 other forms, however, the arrangement (if the plates on the plastron (apart from the 

 labrnni) approaches the nsnal arrangement, i.e. the plates of the two rows alternate 

 more or less distinctly, so that the median sutnre which divides them forms a zig- 

 zag Una. This arrangement, as compared \\ith that first described, is older and more 

 primitive. The test is then called merido sternal. 



In most Clypeastridcc the interambulacra are interrupted, Lc. they do not run 

 continnously from the apical system to the peristome, but, near the latter, are 

 crowded out by the broad plates of the ambulacra which touch one another inter- 

 radially, so that the five interradial marginal plates of the peristome are completely 

 isolated from the remaining portions of the interambulacra (Fig. 306). Not infre- 

 quently, the paired interambulacra are interrupted and the unpaired posterior inter- 

 ambulacrum is uninterrupted. 



[{h) Form of the Peristome. 



In most Echinoidea, i.e. in those in which the peristome retains its central posi- 

 tion, its shape is pentagonal, or decagonal, or round, less frequently oval or oblique, 

 or quite irregular, often with branchial incisions. But wdiere the j^eristome is 

 shifted anteriorly, as in the sub-order SjJatancjoidea, the peristome is transverse and 

 crescent-like, with depressed anterior upper-lip and raised posterior under-lip. The 

 peristome, however, is always central in the embryo, and is originally pentagonal. 



(j) Ornamentation. 



The outer surface of the plates of the Echinoid test are beset — in many diflerent 

 ways, which are of importance in classification — with numerous larger or smaller pro- 

 minences, granules, etc., on which spines and j)edicellariae are planted. 



In the sub-order Spatangoidca, narrow, finely granulated streaks or bands run, in 

 definite arrangement, along the surface of the test, and carry small rudimentary 

 spines or pedicellari.ie. These are called fascicles or Semites (Fig. 302, p. 342). The 

 following systematically important forms of fascicles are to be distinguished : — 



1. The peripetaloid fasciole encircles the apical rosette of petaloids. 



2. The lateral or marginal fasciole runs round the shell near the andjitus. 



3. The lateral subanal fasciole branches off from the peripetaloid fasciole and 

 runs below the anus. 



4. The subanal fasciole forms a ring below the anus (between the latter and 

 the peristome). They may give off anal branches which run up on each side of the 

 anus, and occasionally unite above it to form an anal fasciole. 



5. The internal fascioles run around the apex and the anterior ambulacrum. 

 The tentacles and plates in those regions which are encircled by the internal 



and subanal fascioles are modified. 



One very varied form of ornamentation of the Echinoid test, which arises early 

 during postlai'val development, is due to the deposit of calcai'cous substance on the 

 plates, and is known as epistroma. 



{1-) Marginal Incisions or Perforations. 



These are often to be found in the fiat disc-shaped test of the SeiiteUlda-, in 

 some or all of the ambulacra, and not infrequently also in the posterior inter- 

 ambulacrum. The edge of the shell is at first entire, but during growth marginal 

 indentations and incisions make their appearance, and these may close to form per- 

 forations (lunulse). (Figs. 234, 235, pp. 292, 293, and 306, p. 346.) 



