REPORT ON THE HEXACTINELLIDA. 497 



Jistiuguislied as a separate group, not ouly in the substance out of which the skeleton 

 consists, but also in the form of the component spicules. 



One is therefore inclined to suppose their very early divergence from the great sponge 

 stem. On the other hand, the marked uniformity of their skeletal elements lead one to 

 suppose that they have had a common starting point, i.e., a monophyletic origin. This 

 supposition is confirmed by the certain fact that the Sycones in their ontogenetic 

 development pass through a distinct Ascon stage, and that between the Sycones and 

 Leucones recent investigations have discovered many connecting links, as has been shown 

 especially in the Challenger Eeport on Calcareous Sponges by Polejaeff, and von Lenden- 

 felcVs researches on Austi'alian forms. 



A closer relationship connects the siliceous, horny and soft sponges. In regard to 

 the last I have previously shown, that both on account of the incongruity between the 

 different members of the group, and the obvious relationship between certain forms and 

 indisputable horny and flinty sponges, the group cannot be regarded as independent, 

 closed and natural, but must be split up and its members referred to different positions 

 on the genealogical tree near their various congeners, and regarded as twigs degenerate 

 in respect to their skeleton. 



In regard to the horny sponges there seems to me no other supposition possible, but 

 that of regarding them as originating from flinty or flinty-horny sponges by the gradual 

 reduction and final disappearance of the siliceous spicules. 



The more abundant and differentiated the horny substance the more degenerate the 

 flinty skeleton, until finally, as in many Chalinidee, which approach the true Keratosa, 

 we find only very simple smooth spindles, which I am compelled to regard as the 

 extreme of the phylogenetic series of siliceous s^aicule modification. 



In my memoir on the family of Plakinidse, I have shown in detail why it is that in 

 "the long and continuous series of transitions between the typical regular tetracts and 

 the simple straight spindles, exhibited both in individual species and often in 

 one individual, as weU as in the skeletons of nearly related species, it is impossible 

 to regard the straight spindle as the primitive form from which the triacts and tetracts 

 have been formed by the growth of new rays, but necessary to regard the tetracts as 

 primitive and ancestral, from which the triacts and diacts have arisen by atrophy and 

 degeneration of the various rays. Oscar Schmidt^ was led to the same conclusion by a 

 detailed investigation of other Tetractinellida, and especially of the Ancorinidse, where 

 it may be very readily seen how gradual reduction of the typical tetracts, only modified 

 into anchors by the elongation of one ray, leads finally to simple rod-like spicules, and 

 further how within those genera, which, like Caminus, have acquired rod-like spicules, 

 degenerate anchors here and there persist, showing the mode in which the rods have, 

 originated from tetracts. 



■ Entstehung iieuer Arten durch VerfoU uiid Scliwund alterer Merkniale, Zeitschr.f. wiss. ZooL, Ed. .\lii. p. 039. 



