734 MICEOSCOPIC FORMS OF ANIMAL LIFE -PROTOZOA 



exercise it to find a suitable situation for their attachment, the 

 capture of their food being effected by their pseudopodial net- 

 work. 



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) 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 characterised by the enclosure of 

 symbiotic yellow corpuscles (zoochlorellce) 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 some- 

 times perforated casings of great regularity of form, as in the 

 marine Polycystina, sometimes internal frameworks of marvellous 

 symmetry, as in the marine Rqdiolaria. These two groups, also, 

 will be reserved for special notice, the simple ffeliozoa, which 

 are among the commonest inhabitants of fresh water, furnish- 

 ing 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. The tendency 

 of modern writer sis to separate the Heliozoa, as here understood, into 

 the two groups of Heliozoa (sens, strict.) and Radiolaria, the latter 

 being distinguished by the presence of a central capsule or mass of 

 protoplasm surrounded by a special envelope, the better develop- 

 ment of the skeleton, the greater tendency of the pseudopodia to 

 coalesce with one another, and the not imfrequent presence of ' yellow 

 bodies.' 



III. The third group, Lobosa, 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 endo- 

 sarc, which are continually undergoing -change both in form and 

 number. The Lobosa are comparatively active in their habits, moving 

 freely about in search of food, which is still received into the sub- 

 stance of their bodies through any part of their surface unless this 

 is enclosed 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 aggre- 

 gating and uniting grains of sand or other small solid particles, 

 which they build up into ' tests.' A large proportion of them are 

 inhabitants of fresh water, and some are even found in damp 

 earth. 



Reticularia. This type is very characteristically represented by 

 the genus Gromia (fig. 571), some of whose species are marine, and 

 are found, like ordinary Foramini/era, among tufts of corallines, 

 algae, &c. ; 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 



