862 



SPONGES AND ZOOPHYTES 



T 



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 silicious, 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 

 011 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 



11. ZOOPHYTES (COZLEXTERA) 



Under the general designation Zoophytes it will be still con- 

 venient to group those animals which form composite skeletons or 



4 polyparies ' of a more or less 

 plant-like character, associating 

 with them the Acalej>/<8, which 

 are now known to be the ' sexual 

 zooids' of polypes, but excluding 

 the Pohpoa on account of their 

 very different structure, not- 

 witlistandiiig their zoophytic 

 forms and habits of life. The 

 animals belonging to this group 

 may be considered as formed 

 upon the primitive gastrula 

 type, their gastric 

 (though sometimes 

 itself almost indefinitely) being 

 lined by the original endod&rm, 

 and their surface being covered 

 by the original ectoderm, and 

 these two lamella? not being 

 separated bv the interposition 

 of any body-cavity or ccelmit. 

 It is a fact of great interest 

 that although the product of 

 the development of a iimrnln. is 

 here a distinctly individualised 

 polype, in which several mutu- 

 ally dependent parts make up 

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

 independent protosoic 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 amo-boid cells, which send out pseiidopodial 



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

 Dr. G. (.'. Yosmaer as vol. ii. of I'.ronnV A"/./x.sry/ /mil (>nl>t nffcn des TMerreichs, 

 :ig, 1HS7. Compare also the article by Professor Sollas in the ninth edition of the 



cavity 

 extending 



FIG. 657. Longitudinal section of the body 

 of a hydra killed in full digestion : ec, 

 ectoderm; en, endoderm ; >i/]>, muscular 

 processes; d, a diatom; /, food. (After 

 T. J. Parker.) 



