526 MODES OF MOUNTING POLYCYSTINA. ACAXTHOMETPINA. 



when these objects are examined by light transmitted through 

 them. And when they are mounted in Canada Balsam, the 

 Black-ground illumination, either by the Webster Condenser 

 (§ 80), the Spot Lens (§ 84), or the Paraboloid (§ 85), is much to 

 be preferred for the purpose of display, although minute details 

 of structure can be better made-out when they are viewed as 

 transparent objects with higher powers. Many of the more solid 

 forms, when exposed to a high temperature on a slip of Platinum 

 foil, undergo a change in aspect which renders them peculiarly 

 beautiful as Opaque objects ; their Glassy transparence giving place 

 to an enamel-like opacity. They may then be mounted on a black 

 ground, and illuminated either with a Side-Condenser, or with 

 the Parabolic Speculum (§ 91). — No class of objects is more suit- 

 able than these to the Binocular Microscope ; its Stereoscopic pro- 

 jection causing them to be presented to the mind's eye in complete 

 relief, so as to bring-out with the most marvellous and beautiful 

 effect all their delicate sculpture. * 



404. Acanthometrina. — In this little group, which seems to 

 form a connecting link between the Polycystina and Sponges, the 

 animal is not enclosed within a shell, but is furnished with a very 

 regular Skeleton composed of elongated spines, which radiate in all 

 directions from a common centre (Plate xix., fig. 3). The soft 

 sarcode-body is spherical in form, and occupies the spaces left 

 between the bases of these spines, which are sometimes partly 

 enclosed (as in the species represented ) by transverse projections. 

 The Ectosarc seems to have a more definitely membranous con- 

 sistence than in Actinophrys ; but it is pierced by the pseudopodia, 

 whose convergence may be traced from without inwards, after 

 passing through it ; and it is itself enveloped in a layer of less 

 tenacious protoplasm, resembling that of which the pseudopodia 

 are composed. The Endosarc contains a number of yellow cell-like 

 globules, resembling those of Thalassicolla? (§ 336). As one species, 

 the Acanthometra echinoides, is very common on some parts of the 

 coast of Norway, especially during the prevalence of westerly 

 winds, it might probably be found on the eastern coast of Britain, 

 especially when easterly winds prevail. To the naked eye it pre- 

 sents itself as a crimson-red point: the diameter of the central 

 part of its body being about 6-1000ths of an inch. 



* The general Plan of Structure of the Polycystina, and the signification 

 of their immense variety of forms, are ably discussed by Dr. Wallich, in 

 the "Trans, of the Microsc. Society," N.S., Vol. xiii., p. 75 : but no system 

 of Classification can at present, in the Author's opinion, be regarded as 

 otherwise than provisional. For a fuller description of the Fossil forms 

 of this group, see Prof. Ehrenberg's Memoirs in the " Monatsberichte " 

 of the Berlin Academy for 1846, 1347. and 1850 ; also his ' Microgeologie,' 

 1854; and "Ann. of Nat. Hist.," Vol. xx. (1847).— The best method of 

 separating the Polycystina from the Barbadoes sandstone is described by 

 Mr. Furlong hi the " Quart. Journ. of Microsc. Science," New Ser., 

 Vol. i. (1861), p. C4. 



