<M..VSS I 



ECHINOIDEA 



217 



Cidarid Spines. 

 E, Porocidaris. 



FIG. 349. 

 A, B, Cidaris. D, Acroc'tilm-l*. 



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 articu- 

 lating joint or condi/le (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 indentations 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 (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 peripetalous fasciole 

 follows the margin of the petaloid 

 parts of the ambulacra. The anal 

 fasciole surrounds the anus, and the 

 subanal 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 fascicles, Loven has proposed the name 

 Prymnodetmia; genera without them, and with other fascicles, are Prymnadetes, 

 and those without any fascicles are Adetes. 



Pedicellariae are small, highly specialised spines articulated upon granules, 

 and scattered in between the spines proper over the whole surface of the test. 

 At the end of the stem is a head consisting of two or three pincer-like valves, 

 whose office is to catch up and pass along foreign matter that has become 

 entangled among the tubercles and spines. 



Spheridia are opalescent spheroidal bodies which are either placed visibly 

 upon short stalks, or are partially or even entirely covered by the test. They 

 occur in all Echinoids with the exception of the Cidaridae, belong exclusively 

 to the ambulacra, and are found usually near the peristome. Anatomically 

 the spheridia may be considered as modified spines having sensory functions, 

 probably of taste or of smelling. 



Ontogeny. The early ontogenetic stages of Sea-urchins are similar in many 

 respects to those of Ophiuroids and Star-fishes, but have little in common with 

 the larvae of Crinoids. Certain changes affecting the test during the course 

 of its development are of great morphological interest, such as the formation 

 of tubercles, variations in the number of plates in the Ecliinidae and other of 

 the Regulares, the transformation of simple ambulacra into petaloid in the 

 Echinolampidae ; the migratory character of the periproct in many Cassiduloids, 

 etc. Much light is shed upon these phenomena by comparison with fossil forms, 

 many of which retain permanently features which are now exhibited transitorily. 



