METAZOA 



43 



through the epithdial lining of these ducts. They then hang 

 freely, supported by a short peduncle, into the lumen of the 

 canal (Fig. 36), and are doubtless fertilised in this position by 



Fig, 35. — 1-7, Development of spermatozoa in Symndra raphanus ; 8, mature 

 spermatozoa, after Polejaeff ( x 792) ; 9, a sperm morula in Oscarella lobularis 

 ( X 500); 10, an isolated mature spermatozoon, after Schulze ( x 500). 



spermatozoa carried in by the inhalant 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. 



Fig. 36. 



1. Ovum [Orantia lahyrinthica) 

 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-calcarea 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. Eepro- 

 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. 



