

ZOOLOGY FOR MEDICAL STUDENTS 



L HAF. 



ha complete new cell-outfit such as it had durin 

 ason. 

 ire classified as follows into three main subdivisions : 



I. CALCAREA 



.ubdivision includes the Ascon and Sycon types which have 



en described. It is characterized above all, as indicated by 



the tact that the spicules are composed of calcium carbonate. 



^hape the triradiate spicules are particularly characteristic 



idrs these there arc commonly present straight or curved monaxon 



^picules. and tour-rayed spicules derived from the triradiate by the 



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



:i "f Sponges. A, Calcareous spicules (Lcucosolenia) ; B, siliceous spicules (Hyaloncma; 

 all spcmgine (Pachychalina and Chalina). 



cavity (Fig. 5-S, A ; Fig. 55). The general plan of structure is primarily 

 ih.it ol tlu- Ascon but complication may take place to a less or greater 

 aloiu; tin- lines indicated on p. 122. 



II. HEXACTINELLIDA 



tmrlluU arc lor the most part deep-sea sponges: the 



plan ol' their .structure is somewhat S\ con-like : their most striking 



.Mul that which gives them their name is the nature of their 



latter are composed of clear glassy silica, in the form of 



ecting one another at right angles so as to give when all 



re -,,ii;illy developed a spicule with six equal rays (Fig. 58, B). 



"nrnonly ' S are not developed equally e.g. a single ray 



: or absent as in the right-hand spicule of Fig. 58, \\, or 



one plane may he reduced so as to produce a spicule 



which has the (lecept i\ e appearance of being monaxon. Finally particular 



