48 



THE SPONGES. 





principals may bear 2 or 3 terminals, but 2 is the commoner number. 

 Exceptionally, as in one of the rays of Fig. 9, Plate 5, there is no proper 

 principal, the branching occurring so close to the centrum of the spicule 

 that the terminals are confluent with it. 



A detail of some interest as bearing on the mode of development of 

 the hexaster form is indicated in Figs. 1 and 4, Plate 5. In spicules 

 where the principal ray bears but two terminals, the latter commonly pass 

 into the principal in an asymmetrical fashion. One of the terminals makes 

 a bend at its lower end, thus becoming strongly convex on this part of 

 its outer surface, while the corresponding surface of the other terminal 

 and the adjoining part of the principal form a weakly concave surface. 

 This fact, together with the angles which the several rays make with one 

 another, often suggests that certain, rays represent the primitive hexact 

 rays and that other terminals are produced as lateral branches on these. Much 

 less commonly the two terminals are symmetrically disposed on the prin- 

 cipal, suggesting an early dichotomy, but in such cases the symmetry may 

 have been superinduced on an earlier asymmetry. Where the principal 

 bears 3 terminals, the arrangement is usually symmetrical and gives no 

 hint as to whether the branching had been lateral or not. But exceptional 

 spicules, like that shown in Fig. 5, Plate 5, occur which speak for the 

 lateral origin of the branches, in that two terminals occupy a lateral 

 position on the same side of what seems to be a primitive hexact ray. 



Very frequently, perhaps always, the opposite rays of a diameter branch 

 in planes at right angles to one another, as shown in Fig. 4, Plate 5, — 

 a phenomenon observed by Schulze in the hexasters of several species 



(1887, p. 31). 



A form of discohexaster, Fig. 5, Plate 4, very similar to the corre- 

 sponding spicule of C. latus F. E. Sch. (Schulze, 1887, Plate XXIV.) and 

 C. agassizii F. E. Sch. (Schulze, 1899, Plate VI.) occurs with about the same 

 distribution as in the latter species (Schulze, 1899, p. 38). It is most 

 abundant near the gastral membrane and in the walls of the large efferent 

 canals, less abundant near the dermal membrane and in the walls of the 

 main afferent canals. In this spicule, the principals are smooth, and taper 

 very slightly toward the apex, where they enlarge to form a base for 

 the terminals. These commonly vary in number from 5 to 10, and are 

 arranged in a whorl. Not infrequently, however, spicules are found with 

 more numerous terminals, up to 16, which do not form a whorl but a brush, 





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