No. 3.] DEVELOPMENT OF MARINE SPONGES. 351 
sponge all the tubes are connected together, and the canals 
lying between them (Intercanals, /z. can. in the figure) 
are complicated. Water enters the intercanals through 
the openings on the surface (surface pores, s. /f.), and 
passes into the radial canals through the numerous chamber 
pores: (67-7,): 
The embryology of the Sycons as far as known confirms the 
belief that they are derived from the Ascons. Thus Sycandra 
raphanus passes through a distinctly Ascon phase (Schulze 25), 
the radial tubes appearing later as outgrowths. The actual 
development of complicated intercanals such as those of Ana- 
mixilla has never been witnessed, but a comparison of a large 
number of forms in which the connection between the radial 
canals varies within wide limits, makes it pretty certain that 
the intercanals of a form like Anamixilla are homologous with 
the simple ectodermic spaces between the radial tubes of 
Sycetta or Sycandra ciliata. It is exceedingly probable that 
the actual development of the complicated Sycons will show 
that the radial tubes are in young stages distinct from one 
another, and only later become connected together by bridges 
of tissue in such a way as to form complex intercanals. And 
so, we must at present regard the intercanals of a form like 
Anamixilla as lined with ectoderm. 
Coming now to the Leucons, we find that Polejaeff’s descrip- 
tion of the anatomy of this family accords with their derivation 
from the Sycons quite as well as did Haeckel’s more imagina- 
tive conception of the structure of these forms. Taking one 
of the simplest of Polejaeff's types, let us compare it with a 
Sycon. In Pl. XXV, Fig. 2, is shown part of a transverse 
section of Leucilla connexiva. Such a form is obviously 
derived from a Sycon by the evagination of the wall of the 
paragastric cavity at certain points (%,+). These evaginations 
give rise to numerous diverticula of the central cavity, which 
constitute efferent canals, ef.c. The radial chambers are at 
the same time thrown into groups, each group opening into 
one of the new diverticula. The intercanals (/z. can.) pene- 
trate as before between the several radial chambers, bringing 
water to the chamber pores (c./.), the complexity of their 
