CHAPTER IV 

 PHYLUM PORIFERA 



The members of the Phylum Porifera (Lat. porus, a pore; 

 ferre, to bear) are commonly called sponges. The ordinary bath 

 sponge of commerce is the skeleton of one of these animals. 

 Most sponges live only in salt water. Formerly they were 

 considered plants because of their irregular and plantlike habits 

 of growth. When their animal nature was finally established 

 (about 1857), the problem of their position in the animal series 

 arose. By many authorities they were considered colonial 

 Protozoa allied with the Choanoflagellata (p. 47), but they 

 are now generally classed with the many-celled animals, and 

 placed in a separate group, the Parazoa, as explained on 

 page 24. 



Sponges may be grouped into three classes according to the 

 composition and shape of their skeletal elements (spicules) : — 



Class I. Calcarea (Lat. calcarius, lime) with spicules of 

 carbonate of lime (Fig. 53); 



Class II. Hexactinellida (Gr. hex, six; aktin, a ray) 

 with triaxon spicules of silicon (Fig. 60, e) ; and 



Class III. Demospongi^e (Gr. demos, people; spongos, 

 sponge) usually with spicules of silicon, not triaxon, or with 

 spongin (Fig. 61), or with both spicules and spongin. 



1. Structure of a Simple Sponge — Leucosolenia 



Leucosolenia (Fig. 52) is a sponge which will serve to illustrate 

 the structure of the most simple members of the phylum. 

 It is found growing on the rocks near the sea-shore just below 



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