350 WILSON. [Vo.u. IX. 
Evidence from Comparative Anatomy as to Sponge 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, /.c.) 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 Homoderma 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 Zodlogical Articles by Lankester, 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 primt- 
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. 1, transverse section of Azamixilla torrest). In this 
