CLASSIFICA TION. 



skeleton, the ovum segments equally into a blastula, which is flagellate 

 all over. This free-swimming stage may be invaginated from either 

 pole to form a hemispherical gastrula, which settles mouth downwards. 

 Pores, an osculum, and the mesoglcea are formed as before, and the 

 inner layer becomes folded into flagellate chambers. 



(c) Another type, seen for instance 

 in a horny sponge, Spongelia, results in 

 a flagellate larva, whose cavity is filled 

 up with what may be called gelatinous 

 connective tissue, from which mesoglcea 

 and endoderm are subsequently dif- 

 ferentiated. Such a. larva is called a. 

 parenthymula. 



As these are not all the types of 

 development which occur among sponges, 

 the general fact is impressive, that in 

 this lowest class of Metazoa there has 

 been considerable plasticity in develop- 

 ment. 



Classification. — 



A. Porifera Cakarea, with skeleton of 

 calcareous spicules : — 



Order I. — Homocoela. — Endoderm 



wholly composed of collared 



flagellate cells, e.g. Ascetta, Leuco- 



solenia. 

 Order II. — Heteroccela. — Endo- 

 derm consists of collared flagellate 



cells in radial tubes or chambers, 



and of flat epithelium elsewhere, 



e.g. Grantia, Sycon. 



B. Porifera non-Calcarea, skeleton of 

 silica or of spongin, or of both. 



(i) Hexactinellida, with sexradiate 

 siliceous spicules, canal system 

 usually simple, with Sycon 

 chambers. The members live 

 chiefly in deep water, e.g. 

 Venus Flower- Basket (Euplec- 

 tella) and the Glass-Rope 

 Sponge (ffyaltmema). 

 (2) Monaxonida, with siliceous spicules (which are not quadn- 

 or sex-radiate), or with "horny" skeleton, or with 

 both. 

 Order I.— Monaxona, with spicules only, e.g. Mermaid s Gloves 

 (Chalina oculata), Crumb-of-Bread Sponge (Halichondria or 

 Amorphina panicea), Fresh- Water Sponge (Spongilla). 

 Order II. — Ceratosa, " horny " sponges with or without spicules, 

 e.g. the Bath-Sponge (Euspongia). 



Fig. 52. — Diagrammatic re- 

 presentation of development 

 of Oscarella lobularis. — 

 After Heider. 



BL, Free-swimming blastula with 

 flagella; G., gastrula settled 

 down. 



Next figure shows folding of 

 endoderm {En.)', Ec, ectoderm. 



Lowest figure shows radial cham- 

 bers (R.C.); Mesogloea(^f.); 

 inhalant pore (P.); exhalant 

 osculum (Q.). 



