350 IVILSOX. [Vol. IX. 



Evidence from Comparative Anatomy as to Sfoiigc Phy- 

 logeny. — The strongest evidence offered by comparative 

 anatomy lies in the series of forms, passing by gradations 

 from very simple to complex types, found in the calcareous 

 sponges (Haeckel 8, Polejaeff 20), and in the little group of 

 silicious sponges, the Plakinidae, described by Schulze (26). 

 A comparison of these forms goes to show that the simple 

 Ascon sponge (Olynthus) must be regarded as the ancestral 

 type of the group, and that by the continued folding of the 

 wall of this simple form were produced the more complicated 

 sponges. Further, the exceedingly complex silicious and horny 

 sponges must be interpreted as colonies in which the limits of 

 the individual can in many cases no longer be recognized. 



The calcareous sponges offer a series of increasingly complex 

 forms, which Haeckel divided into Ascons, Sycons, and Leu- 

 cons. Haeckel's views on the relationship of these forms must 

 be in great measure accepted to-day, though in certain respects, 

 especially as regards the anatomy of the Leucons, later re- 

 searches (Polejaeff, I.e.) have shown that he was not always in 

 possession of the real facts of the case. 



The simplest calcareous sponges, or Ascons, which serve as 

 the basis for Haeckel's hypothetical sponge ancestor, the 

 Olynthus, are too familiar to call for any description. The 

 interesting form Honoderma sycandra (von Lendenfeld) may, 

 however, be mentioned, in which the body is surrounded by 

 radial tubes, after the fashion of a Sycandra, but with the 

 difference that the central cavity as well as the radial tubes is 

 lined with collared cells. A figure of this interesting sponge 

 is accessible, in Sollas's article on Sponges in the Encyc. Brit, 

 (or Zo5logical Articles by Lankcster, etc., p. 40). 



Homoderma bridges the way from the Ascon type to the 

 simplest Sycons, in which the radial tubes are distinct from 

 one another. A surface figure of such a Sycon [Sycetta frimi- 

 tiva) is given in Vosmaer(33), Taf. IX, taken from Haeckel's 

 monograph. In the majority of Sycons, however, the radial 

 tubes are not distinct, but are connected together more or less 

 by strands of mesoderm covered with ectoderm (PL XXV, 

 Fig. I, transverse section of Anami.xilla torresi). In this 



