90 PORIFERA [CH. 



In the case of most of the Demospongiae the ciliated cells nearly, 

 but not quite, surround the granular cells, and these last often 

 contain a number of spicules ready formed in a central bundle 

 which are scattered in all directions when the sponge flattens on 

 fixation. Comparing the development of a sponge with that of the 

 planula of a Coelenterate we see that in the first the ciliated cells 

 form the internal layer, in the second the external layer of the 

 adult ; in the first the animal fixes itself by the pole at which the 

 invagination or intucking of the cells destined to form the inner 

 layer takes place, in the Coelenterate at the opposite pole ; so 

 that if Coelenterata and Porifera had an ancestor in common it 

 could only have been an animal like the organism Volvoa, consisting 

 of a single sphere of cells in a word were it living now it would 

 have been classed as a Protozoon. 



The study of the development of sponges like Sycon shows that 

 at first, after the metamorphosis, the sponge has the form of Leuco- 

 solenia, i.e. a simple cylinder lined by choanocytes. The flagellated 

 chambers arise as horizontal cylindrical branches on the primitive 

 chamber and soon become so numerous that their walls come into 

 contact and the afferent or inhalant canals are simply the crevices 

 left between these chambers. As the chambers develop, flattened 

 cells come inwards from the pores and displace the choanocytes from 

 the walls of the central chamber into the flagellated chambers. 



Porifera then may be defined as animals consisting of branch- 

 systems of tubes, the principal openings of which are exhalant, 

 whereas the inhalant openings are minute perforations of the walls. 

 The wall consists of two layers ; some cells of the inner layer have 

 the form of choanocytes, whilst the skeleton consists of siliceous or 

 calcareous needles formed by cells of the outer layer which wander 

 in, or of spongin. There are never any thread-cells or differentiated 

 muscle or well-marked nerve-cells, nor any such organs as tentacles. 



Sponges are by some of the best authorities divided into three 

 main classes, viz. : 



Class I. CALCAREA. 



This group includes all those sponges with calcareous spicules 

 and comparatively large flagellated chambers. 

 It is divided into two main orders : 



Order 1. Homocoela. 



Sponges consisting of tubes lined throughout with choano- 

 cytes. 



