862 



SPONGES AND ZOOPHYTES 



reagents. The latter method is preferable, as it is difficult to free 

 the mineral residue from carbonaceous particles by heat alone. If 

 (as is commonly the case) the spicules are silicioas, the sponge may 

 be treated with strong nitric or nitre-muriatic acid, until its animal 

 substance is dissolved away ; if, on the other hand, they are cal- 

 careous, a strong solution of potass may be employed instead of the 

 acid. The operation is more rapidly accomplished by the aid of 

 heat ; but if the saving of time be not of importance, it is preferable 

 on several accounts to dispense with it. The spicules, when obtained 

 in a separate state, should be mounted in Canada balsam. Sponge 

 tissue may often be distinctly recognised in sections of agate, 

 chalcedony, and other silicious concretions, as will be more fully 

 stated hereafter. 1 



II. ZOOPHYTES (CCELENTERA) 



Under the general designation Zoophytes it will be still con- 

 venient to group those animals which form composite skeletons or 



* polyparies ' of a more or less 

 plant-like character, associating 

 with them the Acalephs, which 

 are now known to be the ' sexual 

 zooids' of polypes, but excluding 

 the Polyzoa on account of their 

 very different structure, not- 

 withstanding their zoophytic 

 forms and habits of life. The 

 animals belonging to this group 

 may be considered as formed 

 upon the primitive yastrula 

 type, their gastric cavity 

 (though sometimes extending 

 itself almost indefinitely) being 

 lined by the original endoderm, 

 and their surface being covered 

 by the original ectoderm, and 

 these two lamella? not being 

 separated by the interposition 

 of an y >ody-cavity or ccelom. 

 It is a fact of great interest 

 FIG. 657. Longitudinal section of the body that although the product of 

 of a hydra killed in full digestion: ec, the development of a morula is 

 ectoderm; en, eiidoderm ', mp muscular here a distinctly individualised 

 processes ; a, a diatom ; j , rood. (Alter , . r i i 



T. J. Parker.) P orv P e > m which several mutu- 



ally dependent parts make up 



a single organic whole, yet these parts still retain much of their 

 independent protozoic life ; which is manifested in two very re- 

 markable modes. In the first place, the digestive sac is observed 

 to be lined by a layer of amoeboid cells, which send out pseudopodial 



1 A complete and valuable handbook to the Sponges has been published by 

 Dr. G. C. Vosmaer as vol. ii. of Bromi's Klassen und Ordnungen des Thierreichs, 

 Leipzig, 1887. Compare also the article by Professor Sollas in the ninth edition of the 



