vin EGHINODERMATA MORPHOLOGY OF SKELETON 349 



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

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

 labrum) approaches the usual arrangement, i.e. the plates of the two rows alternate 

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

 zag line. This arrangement, as compared with that first described, is older and more 

 primitive. The test is then called meridosternal. 



In most Clffljeastridcc the interamlmlacra are interrupted, i.e. they do not run 

 continuously 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. 



(7t) 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 where the peristome is 

 shifted anteriorly, as in the sub-order Spatangoidea, 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. 



(i) Ornamentation. 



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

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

 minences, granules, etc., on which spines and peclicellariae are planted. 



In the sub-order Spatangoidea, narrow, finely gramilated streaks or bands run, in 

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

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

 following systematically important forms of fascioles are to be distinguished : 



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



2. The lateral or marginal fascicle runs round the shell near the ambitus. 



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

 runs below the anus. 



4. The subanal fascicle 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 postlarval development, is due to the deposit of calcareous substance on the 

 plates, and is known as epistroma. 



(k) Marginal Incisions or Perforations. 



These are often to be found in the flat disc-shaped test of the Scidcllida', 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.) 



