INVERTEBRATE ZOOLOGY. 67 



23. Study of a transverse section. Note the radial canals, characteris- 

 tic of the type, the outlines of which form a symmetrical rosette. The canals 

 are lined with endoderm cells of the same sort as those lining the main 

 chambers of the Ascon. Between these are the spicules— broken by the knife 

 in cutting. Can yoii detect any regular arrangement of these ? Notice the 

 long spicules fonning an external cone capping each canal. What is the shape 

 of a canal 'i Is it possible to tell from cross-sections alone 't Why do some of 

 the canals appear more or less filled up with cells ? 



24. Study of a longitudinal section. How do the canals appear in this ? 

 Are they differently shaped in the middle and at the ends of the section. 

 Explain this. 



After the above study, construct a series of diagrams representing the com- 

 plete structure of the sponge, giving the relation of canals and partitions, 

 arrangement of spicules, etc. These are not to be copies of the sections. 

 Sections are generally imi)erfect. cut obliquelj-. torn. etc. A diagram should 

 be without imperfections and should combine the resTilt of all different modes 

 of observation, and thiis be a graphic method of rejjresenting one's knowl- 

 edge. In constructing these diagrams let each student select the view and 

 devices which seem to her the best means of expressing the structure. Thus 

 for instance : a perfect cross-section, wath whole spicules, occupying such 

 positions as will illustrate their plan of arrangement. The effect is height- 

 ened if delicate colors ])e used to represent the different ])arts [cf. Dendy's 

 plates in Quarterly Journal for 1H98]. A radial chamber, drawn as a solid 

 body, and covered with spic-ules in regular arrangement, would exiiress the 

 shape well and explain the different planes seen in the sections. 



Additional .stiidi/ of sponges : — 



25. Spicules may be isolated from a piece of silicious sixmge by teasing 

 or by K-O-H [cf. 4]. The shapes may be studied and their composition 

 tested by HCl. This should not be added to a specimen in K-O-H. as the two 

 reagents will counteract each other. If the fresh water sjionge, SpongiJla, 

 be macerated in K-(3-H, especiallj' in the case of specimens collected during 

 the autumn, gemmules may be isolated as well as the spicules. 



26. Skeleton of ci tiorn sponge. With a sharp razor make several very 

 thin sections of a perfectly dry piece of ])athing .sponge and drop them into 

 a watch crystal of turpentine for five minutes. Select the thinnest one and 

 transfer it to a slide, spreading it out in a drop of turpentine. This structure, 

 which is the part we use, is merely the skeleton of the sponge animal, the cel- 

 lular portions having been removed by maceration or boiling. This skeleton 



