SKELETAL STRUCTURES. (SI 



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, g), whilst inacuate 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, i, 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, g), smooth or more or less spined (Fig. 2, h). In acuate spicules the head 

 is truncate or evenly rounded (Fig. 2, h, n), and the spicule gradually tapers to a 

 pointed extremity, whilst when it tapers abruptly the spicule is conical (Fig. 2, j). 

 From these there is a gradual transition to the spinulate or pin-shaped spicules 

 (Fig. 2, I, m), in which the head of the spicule is variously enlarged and may be either 

 sub-spherical or conical in form. In another form, which has been named tibiella, 

 the spicule is cylindrical or fusiform with inflated ends (Fig. 2, o). In some 

 acuate and cylindrical forms the spicule is moniliform and consists of a series of 

 inflated rings with intervening furrows (Fig. 2, i, p). 



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, q), in others the ends are sharply incurved like those of a 

 clasp-hook (Fig. 2, r), 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, t) or unequal in size. 



2. Tetractinellid Spicules. — The typical spicule of this group has four 

 equal, straight, pointed 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 



