in PHYLUM AND CLASS PORIFERA 113 



sexually by means of ova and sperms. The ovum develops into a 

 ciliated free-swimming larva, which afterwards becomes fixed and 

 develops into the plant-like adult Sponge. 



The Sponges are sufficiently far removed in structure from the 

 rest of the Metazoa to justify us in looking upon them as con- 

 stituting one of the great divisions or phyla of the animal kingdom. 

 At the same time there is so much uniformity of structure within 

 the group that a division into classes is not demanded ; the phylum 

 Porifera contains a single class. 



The class Porifera is classified as follows : 



Sub-Class I. Calcarea. 



Sponges with a skeleton of calcareous spicules, and with com- 

 paratively large collared cells. 



ORDER 1. HOMOCCELA. 



Calcareous Sponges in which the internal lining membrane 

 consists throughout of flagellate collared cells. 



ORDER 2. HETEROCCELA. 



Calcareous Sponges in which the paragastric cavity is lined by 

 flattened cells, the collared cells being restricted to flagellate 

 canals or chambers. 



Sub-Class II. Hexactinellida. 



Sponges with six-rayed, tri-axon, siliceous spicules, and simple 

 canal system represented by unbranched or branched flagellate 

 chambers. 



Sub-Class III. Demospongia. 



Sponges either devoid of skeleton or with spongin fibres alone, 

 or a combination of spongin fibres and siliceous spicules, the 

 latter, when present, never six-rayed ; the canal system of the 

 Rhagon type (p. 119), usually complicated. 



ORDER 1. TETRACTINELLIDA. 

 Demospongia with tetraxon spicules. 



ORDER 2. MONAXONIDA. 

 Demospongia with monaxon spicules. 



ORDER 3. CERATOSA. 



Demospongia with skeleton of spongin fibres without siliceous 

 spicules. 



ORDER 4. MYXOSPONGIA. 



Demospongia devoid of skeleton. 



VOL. i I 



