SPONGES ioi 



enveloped in an organic sheath, easily seen when the spicule is dis- 

 solved by acid. The mineral substance composing the spicule is 

 almost pure calcite, with traces of sodium, magnesium, and sulphates 

 (Ebner). 



Forms of Calcareous Spicules. Three types of spicule occur in 

 calcareous sponges, the entire skeleton being composed of one or 

 more of these types in varying combinations, namely : (a) monaxon 

 ("acerate" or "oxeote") spicules, of the form of a simple rod or 

 needle ; (b) triactinal or triradiate spicules, each with three arms 

 radiating from a centre ; and (c) tetractinal or quadriradiate, con- 

 sisting each of four rays. Of these three types of spicule, the 

 second and third must be classed together, both being often con- 

 sidered as belonging to the tetraxon type ; the triradiates, however, 

 represent the more primitive form, to which, in the case of the 

 quadriradiates, an additional ray has been tacked on. Each quadri- 

 radiate consists of a basal system of three rays, similar in all 

 respects to a triradiate system, and of a fourth, " apical " or " gastral" 

 ray. Hence the term triradiate system may be employed to denote 

 either a triradiate spicule or the three basal rays of a quadriradiate. 

 In considering, therefore, the modifications and variations of the 

 calcareous spicules, the most natural course will be to discuss first 

 the monaxons, then the triradiate systems, and lastly, the gastral 

 rays of the quadriradiates. 



(a) The moriaxon spicules vary very greatly in size. They are 

 sometimes straight (Fig. 72, r), but more often curved (Fig. 72, t, q, s), 

 and always have the two ends unlike. 



(b) The triradiate systems exhibit "modifications of considerable 

 morphological and systematic importance. At the outset it should 

 be remarked that they always lie embedded in the gelatinous tissue 

 of the body wall, with the rays directed more or less tangentially ; 

 and since the sponge surfaces are usually curved, the three rays 

 very rarely lie exactly in the same plane, and are often very strongly 

 bent out of it (Fig. 72, a). Hence, in the following discussion 

 of the numerous modifications of form exhibited by the triradiate 

 systems, each will be considered as seen projected in a plane tan- 

 gential to the body wall at the centre of the spicule. 



The triradiate systems may be quite asymmetrical in form 

 (Fig. 72, p), but they more usually conform to some definite and 

 symmetrical pattern. In the latter case they may be either 

 " regular " or " sagittal." Regular systems consist of three similar 

 rays of equal size meeting at equal angles, so that the spicule is 

 symmetrical about three planes (Fig. 72, b). In sagittal systems, 

 on the other hand, there is but one plane of symmetry, and the 

 spicule exhibits a bilaterally symmetrical form, with two paired 

 lateral rays and an unpaired posterior ray (basal ray, Haeckel). The 

 sagittal form may, however, be produced in one of two ways, which 



