bo 
CLASS I ECHINOIDEA 17 
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 condyle (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, r - 5 . 
and above the latter is usually a milled 
ring or collar, the indentations of which 
are continuous with the striz of the 
shaft. The function of spines is to 





support the test, to aid in locomotion, [v4 : 
and for defence. In rare cases some es Aa 
of the spines are fixed, and arise JX aye 
directly from the test (Podocidaris). pee: ! 











Gls or: 
Cp Sak) 
= 
oes 
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 jperipetalous 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 fascioles, Lovén has proposed the name 
Prymnodesmia ; genera without them, and with other fascioles, are Prymnadetes, 
and those without any fascioles 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 Echinidae and other of 
the fegulares, 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. 
a) 
= 
ROSS 
Res 











Fig. 349. 
Cidarid Spines. A, B, Cidaris. D, Acrocidaris. 
Porocidaris. 
