REPORT ON THE HEXACTINELLIDA. 487 



the connectiou of the axial (■aiials no longer occurs, though secondary development still 

 results in the formation of a connected skeleton, and in which there are no special 

 needles developed besides the regular hexacts. On this phylogenetic theory, Eurete " 

 (which according to Marshall is entirely without isolated siliceous elements) " is of im- 

 portance, while an ontogenetic recapitulation of the development is illustrated ])y the 

 extremely simple embryo of Hyalonema, wdiich I (Marshall) have described." 



From the still extant genus Sclerothamnus, representing the ancestral form, 

 Marshall derives the group of Synauloidse, in which " the lumina of the spicules, as well 

 as the spicules themselves, are continuously connected with one another, so that the 

 whole lattice-work of the sponge is penetrated by a uniformly connected system of tubes." 

 The other modern Hexactinellida are united by Marshall into the group Asynauloidte, 

 " in which the lumina of the stalks of the various spicules are never connected, but each 

 spicule, so far as its central filaments are concerned, forms an independently developed 

 individual. Where a lattice-work is developed, that is exclusively the result of a 

 syncytium formed by the secretion of layers of siliceous material." 



We thus see that Marshall regards the presence of a special form of dictyonal 

 framework as the oldest type of Hexactinellid skeleton, from which the forms described 

 as Lyssacina (with isolated hexradiate spicules) have afterwards developed. In many 

 of the latter a secondary union of the hexradiate spicules again results in the formation 

 of a connected siliceous framework (the dictyonal framework of our Dictyonina), and at 

 first of this alone, as in Eurete, but afterwards also with the association of adjacent 

 isolated spicules. In other derived forms the secondary fusion of the spicules has not 

 occurred, but the form of the isolated spicules has become more or less complicated. 



In 1877, in his Studies on ossil Sponges,^ Zittel apjjlied the results of his investi- 

 gation of abundant palseontological material to the elucidation of the system and 

 phylogeny of the Hexactinellida. His conclusions agree in several points with those of 

 Marshall, but difier not inconsiderably in others. "Were it true," Zittel says [loc. cit., 

 p. 19), "that the compact Hexactinellid skeleton was developed from a special frame- 

 work of delicate protoplasmic strands, then the older fossil forms must necessarily, on 

 Marshall's theory, belong to the Synauloidfe. This is not, however, by any means the 

 case. My researches on fossil forms have shown that the connected lattice-works 

 consist without exception of fused hexradiate spicules, in which the axial canals are indeed 

 frequently apposed and seem to form closed and connected tubes, but are in reality 

 always separate, while they usually lie, as in the living genera Farrea, Eurete, and 

 Aphrocallistes so that the axial filaments of the different hexacts are. seen to be 

 distinctly separate." 



After Zittel had shown that, even in Sclerothamnus, the axial canals of the lattice 

 framework do not form an open anastomosis, but are referable throughout to single 



' Ahhandl. d. 11. CI. k. baier. Alxid. d. U'iss., BcL xiii. Abth. 1. 



