POLYGASTRIA. 



17 



effort, or tlie mere action of the vibratile cilia 

 completes the operation, and the two young 

 animalcules, thus formed, part company and 

 commence an independent existence. 



The direction of the line of separation varies 

 in different species, and even in individuals of 

 the same species (fig. 17. 4, 5, 6, 7, 8) ; some- 

 times it is transverse, sometimes oblique, and in 

 other cases it traverses the long axis of the body, 

 where the form of the animalcule is elongated 

 or oval. This method of reproduction is ex- 

 ceedingly prolific ; for, as each successive gene- 

 ration arrives at maturity in the course of a few 

 hours, and undergoes the same process of divi- 

 sion, it will be found on computation that 

 the progeny derived from a single animalcule 

 ma)', in the course of a single month, amount 

 to many hundred millions in number. 



In the Vorticellae and allied forms supported 

 by rigid or flexible pedicles the fissiparous pro- 

 cess is essentially similar. The adult bell 

 (fig- 9, ) preparatory to its division becomes 

 considerably extended in a lateral direction (b), 

 in which condition the line of fissure is in- 

 dicated, extending from the mouth of the bell 

 to the point of its connection with the pedicle. 

 An indentation soon appears which, progres- 

 sively extending downwards, soon separates 

 the original animalcule into two, both of which 

 are attached to the stem (c, d). In a short 

 time one or both break loose; in the former 

 case the stem survives, in the latter it perishes. 

 The detached bells speedily assume a new 

 form (e,f), and might easily be mistaken for a 

 totally different genus swimming about by 

 means of cilia situated at both extremities of 

 their barrel-like bodies. At last, having found 

 a fit support, they fix themselves to it, the 

 attached extremity becoming gradually elon- 

 gated into a delicate irritable filament similar 

 to that which they possessed prior to the com- 

 mencement of the fissiparous process. 



Gemmiparous reproduction, Besides the 

 above mode of increase, many of the Vorti- 

 cellas and similarly organized forms throw out 

 little gemmae or lateral buds in the same man- 

 ner as the Hydrae and some other Polypes, 

 which, as they advance to maturity, assume the 

 form of the parent stock, from which they at 

 length become detached, or else remain asso- 

 ciated with the original from whence they 

 sprung. 



Sporiferous reproduction. The gastric vesi- 

 cles of the Polygastria occupy but a small 

 proportion of the interior of their minute 

 bodies ; the rest is partially filled up with a 

 granular tissue, which seems made up of nu- 

 cleated cells, or, in other words, ofsporules or 

 spawn, the germs of future progeny ready to 

 be called into active existence when liberated 

 from the nidus in which they were generated. 

 In Kolpoda] cucullus (fig. 18, 3), these spo- 

 rules are represented in the act of becoming 

 discharged from the parent animalculae. 



In many species of animalcules it is easy, 

 with the assistance of a good glass, to per- 

 ceive in the interior of their bodies certain 

 isolated sacculi endowed with very remarkable 

 powers of contraction and of dilatation ; this 



VOL. IV. 



is repeated at regular intervals ; and so great is 

 the contractile force that the little sac seems 

 entirely to disappear, and then in a short time 

 slowly dilating regains its former size. These 

 sacculi Ehrenberg at first thought to be sto- 

 machal cavities, which the creature could 

 alternately fill and empty ; but subsequent 

 observations convinced him that they were or- 

 gans of a peculiar character. By slightly com- 

 pressing large specimens, such as Paramecium 

 aurclia, he further observed that these con- 

 tractile vesicles were generally two (sometimes 

 three) in number, occupying determinate situ- 

 ations in the creature's body, and that from 

 each of these a number (eight) of little canals 

 were given off' like rays from a centre towards 

 the circumference of the body. These canals 

 became gradually enlarged as the sacculus con- 

 tracted ; and vice versa, when the vesicle dilated 

 the canals shrunk and disappeared. Each canal 

 is slightly enlarged at its origin from the central 

 cavity, and the whole apparatus has the appear- 

 ance of two little Op/iiuri, or thin-rayed star- 

 fishes, enclosed in the body of the animalcule 

 (Jig- 18, 1 & 4, s, s). The contractile sacculi 

 were seen by Ehrenberg in at least four-and- 

 twenty different species of Polygastrica;butthe 

 radiating canals were detected in two only, viz. 

 Paramecium and Ophryoglena. 



Fig. 17. 



1, 2. Spirostomum virens. 3. Glaucoma scintillans. 

 4, 5, 6, 7, 8. Glaucoma scintillans in progress of 

 fissiparous reproduction, showing its different modes 

 of fissure. (After Ehrenberg.} 



These organs exhibit, both in their number 

 and situation, important differences in different 

 species. In Paramecium aurelia, Paramecium 

 caudatum, Leucophrys sanguinea, Trachelius 

 anas, Bursaria vernalis, and Stentor Mullen, 

 two of them are found, one situated in the 

 middle of the anterior, and the other in the 

 middle of the posterior, halves of the animal. 

 All the above species, with the exception of 

 Stentor, multiply by spontaneous transverse 

 division, and when thus divided each portion 

 retains one of the contractile organs, and their 

 being thus double seems to have some relation 



