488 THE VOYAGE OF H.M.S. CHALLENGER. 



fused hexacts with blind processes lying side by side, he was led altogether to deny the 

 accuracy of Marshall's group of Synauloidse. He also threw doubt on the correctness 

 of Marshall's representation of Eurete, Semper, as a Monacid. On this account Zittel 

 divides all the Hexactinellida into two main divisions, — Lyssacina, in which " the skeletal 

 spicules generally remain isolated, and only united by protoplasm" and Dictyonina, in 

 which " the skeletal spicules are fused in a regular way, and form a connected lattice- 

 work with cubical or polyhedral meshes. " The soldering of the spicules into a compact 

 connected framework, as occurs in some Lyssacina such as Euplectella aspergillum, 

 cannot be identified in mode or nature of union with the regular fusion of a distinct 

 dictyonal skeleton. The irregularity of arrangement, and the inhibition of further 

 development of the spicules as the consequence of this external union, demonstrate the 

 secondary importance of the former case. 



For the further classification of the Lyssacina, Zittel emphasises, like Marshall, the 

 degree of differentiation exhibited by the loose spicules, and forms three families : — (1) 

 Monacidse (Zittel), with only a single form with loose skeletal elements, including also 

 the few known fossil Lyssacina (like Astrseospoyigia, Eom, and Stcmractinella, Zittel); 

 (2) Pleionacidas, Marshall, in which the main mass of the skeleton consists of regular 

 hexacts, in association with broom-forks and rosettes {Asconema, S. Kent, and 

 Lanuginella, 0. Schmidt); and (3) PoUacidaj, Marshall, in which the form of the skeletal 

 and loose spicules is very manifold, especially in the dermal skeleton and in the lining 

 of the enteric cavities, while the base usually bears a root-tuft of long siliceous spicules 

 (numerous living and some fossil forms). The Dictyonina, on the other hand, are 

 divided by Zittel into a large number of families with complicated characteristics. 



In an essay by Marshall and Meyer ^ on some new or slightly known Philippine 

 Hexactinellids, Marshall withdraws his opinion as to the continuity of the axial canal 

 system in the framework of Sclerothamnus, and thus abandons the group of synauloid 

 Hexactinellids. 



In 1880,^ 0. Schmidt discusses these systematic questions in detail. He says (p. 41) 

 that " the conception which one is apt to associate with the division of the Hexactinellids 

 into Dictyonina and Lyssacina, namely, that each group for itself has originated from one 

 or several common ancestral forms, and that all the Dictyonina, and especially the recent 

 forms, are more closely related to one another than to the Lyssacina, is certainly not 

 in accordance with the facts. The relation between these two divisions appears indeed 

 to be much closer ; the phylogenetic branches have probably repeatedly crossed, and 

 are interlaced by connecting twigs. In one of the new genera, Hertwigia, this insepar- 

 able relationship is expressed in the most convincing way, since this sponge at the 

 branched base is distinctly dictyonal, but forms further up, where it consists of irregular 

 tubes and plates, a transitional type, and finally, still further up and towards the exterior, 



1 Mitth. a. d. k. zoolog. Mus. Dnsden, 1877. - Spongieu des MeerLusens von Mexico. 



