No. 3.] DEVELOPMENT OF MARINE SPONGES. 357 
Plakina trilopha goes a step farther in the direction of com- 
plexity than does Plakina dilopha. It has probably been 
derived from the latter species (compare Figs. 9 and 10) 
by the appearance of secondary folds in the radial efferent 
tubes; by a transformation of the basal cavity into a system 
of lacunae, owing to the increase in number of the connecting 
strands of tissue between the basal layer and the part of the 
sponge containing the flagellated chambers ; and by a compli- 
cation in the afferent canals in consequence of which they do 
not open each by a single aperture but by a number of small 
apertures, the surface pores (s. .). 
Schulze’s conclusion that these species all lie in one line of 
descent, that is that the second has been derived from the 
first, and the third from the second, receives as much support 
from a study of the spicules, as of the canal system. But on 
this head, reference will have to be made to the original paper. 
From comparative anatomy we conclude that the phylogeny 
of the sponges is something as follows: The Olynthus is the 
common ancestor of the group. The outgrowth of radial 
tubes gave rise to the Sycon type. The growth of the meso- 
derm and development of new endodermic diverticula, coupled 
with the metamorphosis of radial tubes into flagellated cham- 
bers, produced the Leucons. The non-calcareous sponges have 
been derived from types with a canal system more or less like 
that of the Leucons. And the conclusion with regard to the 
germ layers is that the efferent system is entirely endodermic, 
and the afferent system entirely ectodermic. 
Embryological Evidence. — Let us see now how far the 
known facts of development support the above conclusions. 
The evidence from the calcareous sponges (Sycandra passes 
through Olynthus stage) has already been given. Several of 
the non-calcareous sponges (Oscarella lobularis, Reniera fili- 
grana, Chalinula fertilis, Plakina monolopha) run through a 
stage known as the Rhagon (Sollas), which it is permissible to 
regard as the ontogenetic representative of the Sycon type. 
The rhagon of Oscarella (Heider 9) is shown in Pl. XXV, 
Fig. 11. Regarding it, as seems best, as equivalent to the 
Sycon type, it will be noticed that the radial tubes of the 
