368 MR. CHARLES STEWART ON THE SPICULA 
The ambulacral tubes are abundantly supplied with spicula, usually triradiate, but 
larger and more irregular towards their free extremity. (Plate XLVIII. fig. 17, a.) 
In the ovary I have as yet been unable to detect any. 
In Zchinometra the spicula are perhaps the most interesting of any, both from their 
great variety and number, and from the transitional forms they present between straight, 
hooked, and triradiate spicula, and perforated plates. 
'The ovary is erowded with bundles of long acerate spicula resembling those of many 
sponges ; from the sides of these frequently project secondary processes, which, reuniting 
to it, convert the spieulum into a perforated plate; bihamate spicula and small plates 
of various forms are also abundantly scattered through its substance. (Plate XLIX. 
figs. 1, 2.) 
In the intestine and its free inner membrane are numerous small plates and hooks, 
varying in their proportionate number in different parts, and in shape and dimensions 
in the species. (Plate XLIX. figs. 3, 4, 5, a, b.) 
The mesentery also is abundantly supplied with spicula, having their long axes in the 
direction of its fibres, and often closely resémbling those of the ovary. (Plate XLIX. 
figs. 3, 4, 5, c.) 
_ The spicula of the ambulacral tubes are generally of the bihamate variety, differing in 
size and amount of curvature in the various species, and having sometimes the form of a 
ring, owing to the union of their points. In one specimen triradiate spicula were found 
at the base of the ambulaeral tubes ; but these may be the spicula of a sponge accidentally 
entangled amongst them, although every available precaution was taken to prevent such 
an accident, and in this species the spicula of the ovary and mesentery presented a more 
than usual tendency to that form. (Plate XLIX. fig. 6.) 
In the genera previously described the principal variations in the form of the spicula 
occurred in the internal parts, those of the ambulacral tubes and branchiæ undergoing 
but few modifications; in the genus Echinus, however, the internal spicula are always 
of the bihamate form, varying only in their size and curvature, and the amount of deve- 
lopment of the central nodule; sometimes, also, one of the hooks is repressed or reversed 
(Plate L. figs. 2, 3, 5, 5, c); they are generally most numerous in the ovaries, pha- 
rynx, and esophagus; in the madreporic canal they are also abundant. 
The spicula of the ambulacral tubes and external branchiæ, from the former of which 
they are sometimes entirely absent, present, however, great diversities of shape and 
dimensions, although in most the bihamate type prevails. Plate L. fig. 2 a is the most 
frequent form; fig. 3a are those of E. Dröbachiensis, described by Valentin under the 
name of E. brevispinosus, and E. neglectus by Forbes; the plane hooks are found im- 
mediately behind the terminal rosette. 
The largest and strangest spicules I have yet found occurred in a small, long-spined 
specimen from India (Plate L. figs. 1, 4), which, although labelled ** Echinus,” evidently 
does not belong to that genus, but seems to answer more to the description of Helio- 
cidaris given by Dujardin and Hupé. 
They are found in the ambulacral tubes, in which they arise as irregular, finely spinous. 
perforated plates, by the thickening of which the perforations are converted into tubes - 
