452 



FORAMINIPEEA, POLYCYSTINA, AND SPONGES. 



Una, especially when seen brightly illuminated upon a black 

 ground; since (for the reason formerly explained, § 62) their 



Fig. 2 is. 



Fi(J. 216. 



Fig. 216. Stijlodidya gracilis. 

 Fig. 216. Asirwiima Ari^totdis. 



"solid forms" become much more apparent than they are when 

 these objects are examined by light transmitted through them. 

 And the "black ground illumination," either by the "spotted 

 lens" or by the "paraboloid" (§ 61), is much to be preferred for 

 this purpose, to the ordinary mode of illuminating opaque objects 

 by incident light from a condenser, although this may be advan- 

 tageously had recourse to, by the Microscopist who is unprovided 

 with these appurtenances. E"o class of objects is more suitable 

 than these to the "Binocular Alicroscope" (§ 40); the stereosco- 

 pic projection of which causes them to be presented to the mind's 

 eye in complete relief, so as to bring out with the most marvel- 

 lous and beautiful effect all their delicate sculpture, reminding 

 the observer (to compare small things with great) of the finest 

 specimens of the hollow ivory balls carved by the Chinese.^ 



295. Sponges. — Although this tribe has been bandied from the 

 Animal to the Vegetable kingdom, and back again, several times 

 in succession, yet its claim to a place among the Protozoa may 

 now be considered as pretty certainly determined, by the infor- 

 mation derived from Microscopic examination of its minute 

 structure. For in the liviiig Sponge, the skeleton, usually com- 

 posed of a fibrous network strengthened by spicules of mineral 

 matter, is clothed with a soft flesh ; and this flesh has been 

 found \>y Dujardin and all subsequent observers to consist of an 

 aggregation of Amceba-like bodies (Fig. 217, b), some of which 

 (as Mr. Dobie has shown)"* are furnished with one or more long 



' For a fuller descriplion of this group, see Prof Ehrenbevs's Memoirs in the " Transac- 

 tions of tlie Berlin Academy" for 184C, 1847, and his recently published" JMiorogeologie ;" 

 also "Ann. of Nat. Hist." 1847. 



2 Goodsir's Annals of Anatomy and Physiology," No. 2, May, IS.3'3. 



