18 OF THE RHIZOPODA IN GENERAL :—RADIOLARIA. 



Order RADIOLARIA. 



12. Actinophryna.* — Commencing our more detailed consideration of these groups with 

 the one which may be regarded as the most characteristic type of the entire series, we find a 

 familiar representative of this in the weW-known Jciinop/uy/s (PI. I, figs. 1 — 4), a not uncommon 

 inhabitant of collections of fresh water in which aquatic plants are growing, and sometimes 

 also presenting itself in the sea. The form of its body usually approaches the spherical (fig. 1) ; 

 but it is liable to modification, on the one hand, from the protrusion of the " contractile 

 vesicle," v, when turgid, on the other, from the projection either of newly ingested food (fig. 2), 

 or of particles that are about to be got rid of as indigestible. A still greater departure from 

 the spherical form is seen when the Jc/i?iop//ri/s is undergoing duplicative subdivision (figs. 3,4). 

 The average diameter of the body is from l-1300th to l-650th of an inch. From every part 

 of its surface rndiating pseudopodia e.xtend themselves, usually exceeding in length the diameter 

 of the body, and tapering gradually from base to apex, but sometimes swelling again at their ex- 

 tremities into enlargements resembling a pin's head. These pseudopodia, however, vary greatly 

 both in number and in length ; sometimes they are partially or almost entirely retracted (a 

 change which may be induced by agitation of the water) ; and they may even disappear 

 so completely, that the animal cannot be distinguished from an Jniwha until they are 

 again put forth. The endosarc is distinguished by the presence of granular particles, fre- 

 quently coloured, which are mostly derived from the matters ingested as food ; and by the 

 nearer approach of its substance to the consistence of a viscid fluid, as is indicated by the move- 

 ments of these granules. It also usually contains numerous "vacuoles," some of which are seen 

 to contain the alimentary matters ingested by the animal, when it is examined soon after feeding. 

 The ectosarc, on the other hand, is more pellucid and of firmer consistence, though not by 

 any means attaining to the tenacity of a proper membranous envelope. There is, however, 

 no definite line of division between the endosarc, and the ectosarc, the one graduating 

 almost insensibly into the other; but it maybe remarked, that the pseudopodia seem to be 

 derived from the ectosarc alone, the endosarc not extending itself into them. They 

 possess, moreover, a degree of consistence which usually prevents them from coalescing when 

 they come into contact with one another; and whenever such a coalescence does take place, it 

 is to a much smaller extent than is common among Foraminifera (^^ 32, 33). A circulation of 

 granules may be seen by attentive observation to take place along the pseudopodia of Acti- 

 nopliri/s ; but it is far less active than that which constitutes so curious a feature in the life- 

 history of the Foraminifera (^ 34). 



13. Although the existence of a "nucleus" in Jctinoplirj/s has been denied, its presence 

 (in certain species, at least) must be regarded as a well-established fiict.f It presents itself 

 as a flattened vesicular body, with a well-defined margin, usually of circular outline, and very 



* Ou tbis family, see .xi, xxii, xxiv, xxv, xxvii, xxxvi, xxxvni, lvi, lxxxii, lxxxiv, cii^, cv. 

 t The term " nucleus " was improperly applied by Kolliker (lvi) to the dark granular sub- 

 stance which coinmonly occupies the central portion of the body. The existence of a true nucleus was 



