CHAPTER III 



PORIFERA 



The phylum Porifera (Sponges) is constituted by a group of organisms 

 comparable in the degree of complexity of their structure with the 

 Coelenterata but which are pretty clearly not Coelenterates. They 

 appear to have arisen as a side branch during the evolution of the animal 

 kingdom and to have sprouted out from the Protozoan stem quite inde- 

 pendently of the Coelenterata. The essential characters of the group 

 are most easily demonstrated by the study of young specimens of the 

 comparatively simple Ascon type of sponge exemplified by the genus 

 Leucosolenia, specimens of which may be found attached to overhanging 

 rocks or stones or seaweeds near low-water mark on almost any coast. 



Leucosolenia 



The Leucosolenia has essentially the character of a tube one or two 

 millimetres in diameter and whitish in colour. The tube is at first simple, 

 as shown in the illustration (Fig. 54), but it becomes complicated by the 

 development of lateral or basal outgrowths which may lead in the first 

 case to the formation of complex tree-like masses, and in the second to 

 the formation of colonies of " individuals " connected by a stolon which 

 creeps over the surface of the substratum. 



At its attached end the tube is closed while at its free end it opens by 

 a wide opening — the osculum (Fig. 54, os). 



The microscopic study of thin sections (Fig. 55) shows the wall of 

 the sponge to be composed of two layers— an inner gastral and an 

 outer dermal (Fig. 55, D). Of these the inner is the simpler for it 

 is composed of a single layer of cells, all alike. These, the choanocytes 

 or " collar-cells," are highly characteristic. Each is somewhat flask- 

 shaped, contains a rounded nucleus rather nearer its free end, and is 

 prolonged into a single powerful flagellum which projects inwards towards 



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