124 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



provide the sponge with a complete new cell-outfit such as it had during 

 the preceding season. 



The Porifera are classified as follows into three main subdivisions : 



L Calcarea 



This subdivision includes the Ascon and Sycon types which have 

 already been described. It is characterized above all, as indicated by 

 its name, by the fact that the spicules are composed of calcium carbonate. 

 As regards shape the triradiate spicules are particularly characteristic 

 but besides these there are commonly present straight or curved monaxon 

 spicules, and four-rayed spicules derived from the triradiate by the 

 addition of a small fourth ray which projects inwards into the gastral 



Skeleton of Sponges, A, Calcareous spicules {Leucosolenia) ; B, siliceous spicules [Hyalonema ; 

 C, siliceous spicules with spongine {Pachychalina and Chalina). 



cavity (Fig. 58, A ; Fig. 55). The general plan of structure is primarily 

 that of the Ascon but comphcation may take place to a less or greater 

 extent along the lines indicated on p. 122. 



II. Hexactinellida 



The Hexactinellids are for the most part deep-sea sponges : the 

 general plan of their structure is somewhat Sycon-like : their most striking 

 feature and that which gives them their name is the nature of their 

 spicules. These latter are composed of clear glassy silica, in the form of 

 three axes intersecting one another at right angles so as to give when all 

 three axes are equally developed a spicule with six equal rays (Fig. 58, B). 

 Very commonly the six rays are not developed equally — e.g. a single ray 

 may be reduced or absent as in the right-hand spicule of Fig. 58, B, or 

 the four rays in one plane may be reduced so as to produce a spicule 

 which has the deceptive appearance of being monaxon. Finally particular 



