

in PHYLUM AND CLASS POR1FERA 103 



of spicules of carbonate of lime. Reproduction takes place both 

 asexually by the formation of getiimules, and 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 their 

 nearest allies the Protozoa on the one hand and the Coelenterata 

 on the other to justify us in looking upon them as constituting 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 endoderm consists throughout 

 of flagellate collared cells. 



ORDER 2. HETEROCCELA. 



Calcareous Sponges in which the endoderm consists partly of 

 flattened cells, the collared cells being restricted to flagellate canals 

 or chambers. 



Sub-Class II, Non-Calcarea. 



Sponges in which the skeleton is either absent, or composed of 

 spongin fibres, or of siliceous spicules. 



TRIBE LMYXOSPONGI^E. 



Non-Calcarea devoid of skeleton. 



TRIBE II.SILICISPONGIdE. 

 Non-Calcarea provided with a skeleton. 



ORDER 1. HEXACTINELLIDA. 

 Silicispongioe with six-rayed siliceous spicules. 



ORDER 2. DESMOSPONGLE. 

 Silicispongise devoid of six -rayed spicules. 



Systematic Position of the Example. 



Sycon gelatinosum is one of many species of the genus Sycon. 

 Si/con is one of several genera of the family Syccttidce ; and the 



