SKELETAL STRUCTURES. 67 
of the spicule and is either open or enclosed by the spicular wall at one or both 
ends. In the case of many acerate spicules, the canal opens to the exterior at 
both ends of the spicule (Fig. 2, a); in cylindrical spicules it is usually enclosed 
at both ends by the spicular wall (Fig. 2, 7), whilst in acuate and spinulate spicules 
the canal is enclosed at the truncate or inflated head and opens at the apex of 
the spicule (Fig. 2, m). In some forms, which are regarded by Zittel as immature 
spicules, the canal remains open throughout its length (Fig. 2, f). In moniliform 
spicules like those shown in Fig. 2, 7, p, the canal does not follow the inflations of 
the spicule, but continues of an even width throughout. 
There is very great variation in the form and size of monactinellid spicules ; 
some of the commoner types are represented in the accompanying Fig. 2. The 
most abundant form is the acerate, generally thickest in the central portion and 
gradually tapering to the ends (Fig. 2, a, b). It may be either straight, curved, 
or vermiculate (Fig. 2, c, d), either pointed or obtusely blunted; smooth or 
partially or entirely microspined. Numerous gradations exist between the 
acerate and the cylindrical forms, which likewise may be straight or curved 
(Fig. 2, 7), smooth or more or less spined (Fig. 2,/). In acuate spicules the head 
is truncate or evenly rounded (Fig. 2, /, 7), and the spicule gradually tapers to a 
pointed extremity, whilst when it tapers abruptly the spicule is conical (Fig. 2, 7). 
From these there is a gradual transition to the spmulate or pin-shaped spicules 
(Fig. 2, /, m), in which the head of the spicule is variously enlarged and may be either 
sub-spherical or conicalin form. In another form, which has been named tibiella, 
the spicule is cylindrical or fusiform with inflated ends (Fig. 2, 0). In some 
acuate and cylindrical forms the spicule is moniliform and consists of a series of 
inflated rings with intervening furrows (Fig. 2, 7, 7). 
There is a still greater range of variation in the form of the minute flesh- 
spicules associated with the monactinellid skeletal-spicules; some are simply 
bihamate (Fig. 2, g), m others the ends are sharply incurved lke those of a 
clasp-hook (Fig. 2, 7), and there is a trenchant inner edge; in a third form 
both ends of the spicule are expanded so that it may be termed bi-spatulate 
(Fig. 2, s), whilst in the very common “anchor” flesh-spicules there are from 
two to three in-curved grapnel-like hooks at both ends of the spicule, which 
may be either equal (Fig. 2, ¢) or unequal in size. 
2. TETRACTINELLID Spicutes.—The typical spicule of this group has four 
equal, straight, pomted rays extending from a common centre, so as to form the 
four axes of a regular three-sided pyramid, the angle between each of the rays thus 
being 120°. A longitudinal canal is present in each ray, extending from the 
centre to the termination of the ray (Fig. 3,a). The modifications of this ground- 
form are exceedingly numerous. ‘Thus the rays of the spicule may be unequal in 
length, and one or all of them may be furcate, as in some of the spicules of 
