8 THE MICROSCOPE AND ITS REVELATIONS. 



a calcareous cement exuded from the sarcodic body. These testaceous 

 forms, which are exclusively marine, constitute the group of Foramini- 

 fera; whose special interest to the microscopist entitles it to separate 

 consideration (Chap. xn.). And it is only for convenience, that two 

 Reticularia which inhabit fresh water also, and the envelopes of whose 

 bodies are usually membranous, are here separated from the Foraminif era 

 (to which they properly belong) for description as types of the group. 

 The Reticularia have little locomotive power, and only seem to exercise 

 it to find a suitable situation for their attachment; the capture of their 

 food being effected by their pseudopodial network. 



ii. The Second division, Heliozoa * consists of the Rhizopods whose 

 pseudopodia extend themselves as straight radiating rods, having little 

 or no tendency to subdivide or ramify, though they are still sufficiently 

 soft and homogeneous (at least in the lower types, 399), to coalesce 

 when they come into contact with each other. These have usually 

 (probably always) a contractile vesicle as well as a nucleus; and the 

 higher forms of them are characterized by the inclosure of peculiar 

 yellow corpuscles (whose import is unknown) in the substance of their 

 endosarc. By far the larger number of this group also have skeletons of 

 Mineral matter, which are always siliceous; and these are sometimes 

 perforated casings of great regularity of form, as in the marine Poly- 

 cystina ; sometimes internal frameworks of marvellous symmetry, as in 

 tne marine Radiolaria. These two groups, also, will be reserved for 

 special notice (Chap, xii.); the simple Heliozoa which are among the 

 commonest inhabitants of fresh water, furnishing the best illustrations 

 of the essential characters of the type. They seem for the most part to 

 have but little locomotive power, capturing their prey by their extended 

 pseudopodia. 



in. The Third group, Lolosa, contains the Rhizopods which most 

 nearly approach the condition of true Cells, in the differentiation of their 

 almost membranous ectosarc and their almost liquid endosarc, and in the 

 non-coalescence of their pseudopodial extensions, which, instead of being 

 either thread-like or rod-like, are lobate, that is, irregular projections of 

 the body, including both ectosarc and endosarc, which are continually 

 undergoing change both in form and number. The Lobosa are com- 

 paratively active in their habits, moving freely about in search of food, 

 which is still received into the substance of their bodies through any part 

 of their surface, unless this is inclosed in envelopes, such as are formed 

 by many of them, either by exudation from the surface of their bodies of 

 some material (probably chitinous) which hardens into a membrane, or 

 by aggregating and uniting grains of sand or other small solid particles, 

 which they build up into ' tests/ A large proportion of them are inhabi- 

 tants of fresh water, and some are even found in damp earth. 



397. Reticularia. This type is very characteristically represented by 

 the genus Gromia (Fig. 283); some of whose species are marine, and are 

 found, like ordinary Foraminif era, among tufts of Corallines, Algae, etc. ; 

 whilst others inhabit fresh water, adhering to Confervse and other Plants 

 of running streams. It was in this type that the presence of a nucleus 

 formerly supposed to be wanting in Reticularia generally, was first estab- 



1 To this group the Author formerly extended the name Radiolaria given by 

 Miiller to one section of it; but he now thinks it preferable to employ the general 

 term Heliozoa given to it by Herjwig and Lesser, restricting the term Radiolaria 

 to the group to which it was originally applied. 



