438 RHizopoDA eeticularia: GROMIA. 



its margin, sometimes from the midst of its ramifications, whilst 

 others are retracted. Not unfrequently it happens that to a spot 

 where two or more filaments have met, there is an afflux of the 

 Protoplasmic substance that causes it to accumulate there as a 

 sort of secondary centre, from which a new radiation of filamentous 

 processes takes place, just as in Fig. 232. The entire absence of 

 differentiation in the Protoplasmic substance, the freedom of the 

 mutual inosculation of its Pseudopodial extensions, and the active 

 cyclosis incessantly going on between these and the body, are 

 three mutually- related conditions, which not only serve to 

 characterize the group of Animals that exhibits them, but to 

 differentiate that group from others. There is, moreover, a nega- 

 tive character of much importance, which is naturally associated 

 with the absence of differentiation, — namely, the deficiency of 

 the ' Nucleus ' and of the ' Contractile Vesicle' that present them- 

 selves alike in the Radiolaria and in the Lobosa. 



327. It is by Animals belonging to this Order, that those very 

 remarkable minute Shells are formed, which are known under the 

 designation Foraminifera. These constitute a group of organisms 

 altogether so peculiar, and presenting so many features of interest, 

 as to call for a somewhat detailed account of them, which will be 

 most conveniently given in a separate Chapter (Chap. x.). 



328. In Gromia, however, we have an example of a Rhizopod 

 which very characteristically exhibits the Reticularian type in the 

 disposition of its pseudopodia (Fig. 232), but which, according to 

 Dr. Wallich (Op. cit. p. 60), possesses both Nucleus and Contractile 

 Vesicle, and thus shows a transition to the higher oi'ders. The 

 sarcode-body of this animal is enclosed in an egg-shaped brownish- 

 yellow membranous ' test,' which seems to be composed of the 

 horny substance termed Chitine ; and this has a single round 

 oi'ifice, whence issue very long pseudopodia that spread at their 

 base over the external surface of the ' test' so as to form a con- 

 tinuous layer, from any poi'tion of which fresh pseudopodia may 

 extend themselves. The smooth coloured ' test ' of Gromia, which 

 commonly attains a diameter of from l-10th to l-12th of an inch, 

 looks to the naked eye very much like the egg of a Zoophyte or 

 the seed of an aquatic Plant ; and its real nature would not be 

 suspected until, after an interval of rest, the animal begins to 

 creep about by means of its pseudopodia, and to mount along the 

 sides of the glass vessel that contains it. Some Gromice are 

 marine, and are found among tufts of Corallines and Alga? ; whilst 

 others inhabit fresh water, adhering to Conferva? and other plants 

 of running streams. 



329. Radiolaria. — A characteristic example of this Order is 

 presented by the A ctinophrys sol (Fig. 233), a minute creature 

 which is not uncommon in ponds and lakes, occurring for the most 

 part amongst Conferva? and other aquatic Plants, and which may 

 be distinguished with the naked eye as a whitish-grey motionless 



