696 



Div. 4. RADIATA.— HOMOGENEA. 



of the group will have to undergo a similar transference ; but it would be premature to attempt the 

 separation at present. 



The forms presented by these Animalcules are extremely various. In some we can scarcely detect 

 any definite shape ; their bodies appear composed of a mass of gelatinous matter destitute of any 

 sohd support, which may project itself into almost any figure. In others there is still a considerable 



variety in the forms assumed by the same 

 individual under different circumstances (Fig, 

 626, d) ; but still a prevailing shape can be re- 

 cognized. In others, again, the body, although 

 still unprotected by any firm envelope, appears 

 to undergo little change in figure, except when 

 afiected by temporary pressure. But there are 

 many species which cannot be influenced even 

 by this; their soft bodies being enclosed in a 

 delicate but firm integument, strengthened by 

 a deposit of siliceous matter. These are termed 

 Fig. 2i.-VARious Forms OF ANIMALCULES. " loricated Infusoria; and their envelopes are 

 often preserved after the death of the Animalcules, accumulating by their multiplication into vast 

 masses. In these loricated tribes, however, were included many forms now transferred to the Vege- 

 table kingdom. 



Those seem most entitled to the designation Animalcules, which have a distinct mouth, surrounded 

 by vibratile cilia. By the agency of these cilia, food is taken into the cavity of the body ; and refuse 

 matter is sometimes expelled through a separate orifice. When these Animalcules have been allowed 

 to remain for a short time in water, in which finely-divided particles of colouring matter are sus- 

 pended, the whole of the transparent body is seen to be studded with coloured globules of a tolerably 

 uniform size, each of them composed of an aggregation of particles. 



From this class of facts it has been inferred by Professor Ehrenberg that a large number of globular 

 cavities exist in the substance of the body, into which the food is received; and he considers that 

 these all sometimes communicate directly with the mouth; but that in general they are arranged 

 along an intestinal tube, into which they open by a short neck. Notwithstanding the high authority, 

 however, which Prof. E. has acquired from his patient and long-continued study of these Animalcules, 

 this doctrine has not received very general assent from those most competent to judge of its merits, 

 being regarded as rather an hypothesis founded on observations, than as itself entitled to rank as 

 an observed fact. 



It is to the action of the vibratile cilia, also, that the great variety of movements exhibited by these 

 beings is entirely due ; and this fact would seem to indicate that these movements are not directed by 

 consciousness. No organs of sensation have been shown to exist in this class of Animalcules ; nor 

 has any indication of a nervous system been discovered. 



Several modes of propagation are seen in this class of Animalcules. Not unfrequently we observe 

 a reproduction by the development of gemmce or buds, as in the Vorticella (Fig. 22, al ; but in other 

 species the multiplication is effected by the separation of the body into two parts, the division some- 

 times taking place longitudinally, as at b, sometimes transversely, as at c. This process takes place 

 with such rapidity under favourable circumstances, that it has been calculated that from a single Pa- 

 rameeciiim (Fig. 22, e), no fewer than 268,000,000 might be produced in a month. In other tribes, 

 however, propagation takes place by ova or germs evolved within the body of the parent, the greater 

 part of whose bulk is often made up of them. 



When the gemmsB remain adherent, instead of becoming detached, compound structures are pro- 

 duced, more or less rfsembling those of Zoophytes in miniature. The groups of Vorticella are 

 examples of this; but a still more remarkable instance is the Volvox, formerly designated the globe- 

 animalcule, but now known to be composed of a congeries of Monads produced by gemmation from a 

 single individual, and invested by a common envelope. 



The classification of this group proposed by Ehrenberg is principally founded on the various forms 

 under which he believes the alimentary canal to exist in them ; and until the existence of these phall 

 be generally admitted, the classification cannot be regarded as having any permanent value. 



