METAZOA 43 
through the epithelial lining of these ducts. They then hang 
freely, supported by a short peduncle, into the lumen of the 
canal (Fig. 56), and are doubtless fertilised in this position by 
Fic. 35.—1-7, Development of. spermatozoa in Sycandra raphanus; 8, mature 
spermatozoa, after Polejaeff (x 792); 9, a sperm morula in Oscarella lobularis 
(x 500); 19, an isolated mature spermatozoon, after Schulze ( x 500). 
spermatozoa carried in by the inhalent current from another 
sponge. After fertilisation they must return back into the 
mesoderm, where the larva develops. <A similar migration 
of ova has been described by Weismann in many Hydroids. 
Fic. 36. 
1. Ovum (Grantia labyrinthica) 
hanging in lumen of inhalent canal. 
2. Epithelium lining inhalent canal. 
After Dendy. 
The early stages of development take place whilst the ovum 
is in the mesoderm; and in the Non-calearea a special invest- 
ment of epithelial cells lines the cavity in which the embryo lies. 
Many sponges are hermaphrodite, and then the spermatozoa 
usually mature before the ova; others are unisexual. Repro- 
duction by gemmation occurs in some species, buds being 
formed which, as a rule, include portions of all the three 
layers; these separate from the parent form, and grow into 
new sponges. In Spongillidae, the only freshwater family, 
resting buds or gemmules are produced, and their production 
involves the death of the sponge. The gemmules consist. of 
certain yolk-bearing cells enclosed in a complicated capsule, 
which serves to protect the cells until the external circum- 
stances permit them to emerge and grow up into a new sponge. 
