SPONGES 107 



work. In Petrostroma, however, the fibres are entirely confined to a 

 relatively thin outer " covering layer," which perhaps rei^resents more 

 than the cortex ; and the greater portion of the sponge body is occupied 

 by a continuous skeleton framework made up of quadriradiates fused 

 together by secondary deposits of calcite ; a type of skeleton not known 

 to occur in any other calcareous sponge, recent or fossil. 



Phylofjeny of Calcareous Spicules. — The triradiates with sagittal angles 

 occurring in Leucosolenia and tlie greater number of Heterocoela are 

 spicules morphologically of a different type from the e<iuiangular 

 triradiates of Clathrinulae and a few Heterocoela. In the Clalhrinidae 

 the triradiates are the first spicules to appear, and each is .shown by 

 the development to be formed by fusion of three monaxons, a fourth 

 being added in the case of quadriradiates. "Wlien independent monaxons 

 occur in this family, they would appear to owe their origin entirely to 

 modification of triradiates (secondary monaxons). In Leucosoleniidae, on 

 the other hand, the first spicules to appear are true (primary) monaxons, 

 each secreted bj' a single cell. The triradiates in this family appear 

 later than the monaxons, and the posterior ray develops at first much 

 more rapidly than the lateral rays. 



In the Heterocoela the origin of the spicules is less known, but 

 has been studied in Sycon by Maas. The greater number of 

 Heterocoela resemble the Leucosoleniidae more closely than the ClatJbrinidae 

 in both skeleton and canal system. 



Histology. — The description given above of the structure of the 

 Olynthus may be taken as representing the main traits in the histology 

 of the Calcarea generally. It is not necessary to do more here than to 

 describe the development of the three-rayed and four-rayed spicules of 

 Clatkrinidae, interesting as instances of compound spicular systems derived 

 from more than one mother cell. Each ray has its own scleroblast, or 

 actinoblast, as it may be termed. 



To form a triradiate spicule three cells migrate into the parenchyma 

 from the dermal epithelium and become arranged in a trefoil-like figure 

 (Fig. 75, 1). The nucleus of each cell then divides into two, in such a 

 way that one nucleus is placed more deepl}' and one more superficially. 

 Between each pair of sister nuclei a minute spicule ray appears, the three 

 rays being at first distinct from each other, but soon becoming united at 

 the centre of the system (Fig. 75, 2). As the rays grow in length the 

 protoplasm of each actinoblast becomes aggregated round each of the two 

 contained nuclei, and finally more or less completely segmented off to form 

 two formative cells, of which the one placed more internally travels to the 

 tip of the spicule ray, while the other remains at the base (Fig. 42, B, 

 h.f.c). The apical formative cell {ap.f.c) sooner or later disappears, return- 

 ing, apparently, to the eijithelium. Tlie basal formative cell (JKf.c) remains 

 at the base of the ray (Figs. 42, B, and 75, 3) until this portion is secreted 

 to its full thickness. It then migrates slowly outwards along the ray, and 

 in the fully formed spicule is found adherent to the extreme tip (Fig. 

 42, B, sp.c). In the formation of a quadriradiate spicule in the 

 Clalhrinidae, the three basal rays are formed exactly as has been described 

 for the triradiates. Each quadriradiate spicule represents, in fact, a 



