122 THE MICROSCOPE AND ITS KEVELATIONS. 



sections of the substance of the sponge. It is by such sections alone that 

 the internal structure of Sponges, and the relation of their spicular and 

 horny skeletons to their fleshy substance, can be demonstated. They are 

 best made by the imbedding process ( 189, 190). In order to obtain 

 the spicules in an isolated condition, the animal matter must be got rid 

 of, either by incineration, or by chemical reagents. The latter method 

 is preferable, as it is difficult to free the mineral residue from carbonace- 

 ous particles by heat alone. If (as is commonly the case) the spicules 

 are siliceous, the Sponge may be treated with strong nitric or nitro- 

 muriatic acid, until its animal substance is dissolved away; if, on the 

 other hand, they be calcareous, a strong solution of potass may be em- 

 ployed 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 recognized in sections of Agate, 

 Chalcedony, and other siliceous concretions, as will be more fully stated 

 hereafter ( 699). 



ZOOPHYTES. 



513. Under the general designation Zoophytes it will be still conve- 

 nient to group those animals which form composite skeletons or ' poly- 

 paries ' of a more or less plant-like character; associating with them the 

 Acalephs, which are now known to be the ' sexual zooids/ of Polypes 

 ( 518); but excluding the Polyzoa (Chap, xv.) on account of their truly 

 Molluscoid structure, notwithstanding their Zoophytic forms and habits 

 of life. The animals belonging to this group may be considered as 

 formed upon the primitive gastrula type (391): 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 lamellae not being separated by the interposition 

 of any body-cavity or ccdom. It is a fact of great interest, that although 

 the product of the development of a morula is here a distinctly indivi- 

 dualized Polype, in which several mutually dependent parts make up a 

 single organic whole, yet that these parts still retain much of their inde- 

 pendent Protozoic life; which is manifested in two very remarkable 

 modes. In the first place, the digestive sac is observed to be lined by a 

 layer of amoeboid cells, which send out pseudopodial prolongations into 

 its cavity, by whose agency (it may be pretty certainly affirmed) the 

 nutrient material is first introduced into the body-substance. This 

 was first noticed by Prof. Allman in the beautiful Hydroid polype 

 Myriothela; 1 the like has been since shown by Mr. Jeffery Parker to be 

 true of the ordinary Hydra? and Prof. E. Eay Lankester has made the 

 same observation upon the curious little Medusa lately found in a fresh- 

 water tank. 3 (It may be mentioned in this connection, that Metschni- 

 koff has seen the cells which line the alimentary canal of the lower 

 Planarian worms gorging themselves with colored food-particles, exactly 

 in the manner of Amcebcz.) The second ' survival ' of Protozoic inde- 

 pendence is shown in the extraordinary power possessed by Hydra, Acti- 

 nia, etc., to reproduce the entire organism from a mere fragment 



1 "Philos. Transact.," 1875, p. 552. 



2 " Proceed, of Roy. Soc.," Vol. xxx. (1880), p. 61. 



8 " Quart. Journ. Microsc. Sci.," N.S., Vol. xx. (1880), p. 37L 



