SPONGES {Porifera) 



As different spegies of Sponges can usually be obtained in the fresh con- 

 dition for examination, no particular type has been selected for description ; 

 but a general account of sponge structure is given as a guidq to the practical 

 examination of the type available. 



Form and Qeneral Structure. 



Sponges are sedentary (except in the larval state), and live attached to 

 the surfaces of rocks, weeds, etc. They are aquatic and mostly marine, and 

 they have a plant-like appearance. Some are solitary, many are colonies of 

 "individuals " closely grouped or compacted together. These colonies are 

 formed by vegetative budding from a creeping stolon, and become more or 

 less complex by branching. The great differences in size, form and appear- 

 ance among sponges are largely due to the nature of the budding. The 



Embryo 



Endoderm 



Choanocytes 



Spicules 



Spicules 



Mesogloea 



Ectoderm 



Fig. 4. — Transverse Section through a part of the wall of Sycon gelatinosum. 

 After Parker and Has well. 



simplest types are tubular in form [e.g., Leucosolenia, a simple Ascon sponge), 

 or sac-shaped {e.g., Grantia, a Sycon sponge). 



The body of a sponge is a system of canals or waterways ; the water passes 

 in through numerous minute inhalent pores in the body-wall, and, after 

 circulating throughout the internal cavity or cavities, it passes out at one or 

 several large exhalent openings or oscula. The chief internal differences in 

 sponges are seen in this canal system, of which there are distinct types. 

 Otherwise the structure of the body is similar in all the sponges. 



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