268 



ECHINODERMATA— ECHINOZOA 



PHYLUM IV 



t 



m 



Fig. 375. 



Cidarid Sijines. A, B, Cidaris. D, Acrocidaris. 

 E, Forocidaris (natural size). 



central foramen for a slight distance, or imperforate when it is not, A plain 



or sunken space surrounding the base of the tubercle is called the scrobicule, 



or areola ; its outer limit, the scrohicular circle, is generally marked by a ring 



^ D £ ^^ granules, but in many cases the 



T scrobicules of the same meridian are 



continuous. Secondary tubercles may 

 or may not be scrobiculate. 



All the tubercles of Echini bear 

 movable spines, which differ greatly in 

 dimensions, and in the shape and' 

 nature of their cross-sections. They 

 are usually cylindrical, acicular, clavate 

 or spatulate in form, and consist of 

 the following parts : A more or less 

 elongate distal portion or shaft ; a base, 

 to which ligaments are attached for 

 keeping the spine in place ; and an 

 articulating joint or conchjle {acetabulum), 

 forming a ball-and-socket joint with 

 the tubercle proper. When the base 

 of the mamelon is crenulated, the base 

 of the spine is incised in the same 

 manner, and above the latter is usually a milled ring or collar, the indenta- 

 tions of which are continuous with the striae of the shaft. The function of 

 spines is to support the test, to aid in locomotion, and for defence. In rare 

 cases some of the spines are fixed, and arise directly from the test (Recent 

 Podocidaris). 



Fascioles are narrow bands of close granular ornamentation which 

 support rudimentary spinules and pedicellariae. They occur only in the 

 Spatangoids, and are restricted to certain parts of the test. The peripetalons 

 fasciole follows the margin of the petaloid parts of the ambulacra. The 

 anal fasciole surrounds the anus, and the suhanal fasciole encloses a space or 

 plastron beneath the anus, but may send anal branches upward. The internal 

 fasciole crosses the ambulacra at a variable distance from the apical system, 

 and the marginal fasciole encircles the test above the ambitus. For those 

 Spatangoids with subanal fascioles, Lov6n has proposed the name Prymno- 

 desmia ; genera without them, and with other fascioles, are Pnjmnadetes, and 

 those without any fascioles are Adctes. 



Pedicellariae are small specialised organs articulated upon granules and 

 scattered between the spines over the whole test. At the end of the stem 

 is a head consisting of two or more pincer-like valves which function as 

 grasping and cleansing organs. Pedicellariae are very rarely preserved in 

 the fossil state. 



Sphaeridia are opalescent spheroidal bodies which may be partially or 

 entirely covered by the test. They exist singly near the ventral border of 

 the ambulacra, or if moi:e than one, the series extends dorsally, even to the 

 ocular plate. Morphologically, sphaeridia may be considered as modified 

 spines having sensory functions. They are known only in the Centrechinoida 

 and Exocycloida. 



Ontogeny. — The early larval stages of Echini are similar in many respects 



