214 GE>^ERAL HISTORY OF THE INFUSORIA. 



lleproduction by positive complete fission is opposed by the existence of 

 the shell, which is a product from the siuface of the animals adapted to their 

 outline, and increasing only in proportion with the augmentation of the 

 animal substance. 



The cohesion of two or even more ArcelUna by means of their gelatinous 

 substance, and often with the near approximation of the orifices or mouths 

 of their shells, has been remarked by many observers, who for the most part 

 have pronoimced it a sort of " conjugation," a true rejH'oductive act. Oohn 

 has indeed designated it " copulation," and states it to be a general phe- 

 nomenon among Ehizopods. He afih^ms that he has many times seen two 

 Difflugice with the mouths of their shells so firmly connected, that strong 

 shaking of the water about them failed to detach them ; and that likewise 

 one shell was often empty, and the contents of the two aggregated into a 

 globular mass in the other. Leclere, the fii'st describer of Difflugice, in 1815, 

 noticed a like cohesion between two individuals of Difflugia HeJiv ; and Cohn, 

 moreover, is able to confirm the fact represented by Perty of the cohesion of 

 a brown and of a pale shell together. 



Schneider has likewise noticed this adhesion of two animals, and thus 

 speaks of it : — " Tnie double animals of Diffiugia Enchelys are frequently 

 met with (XXI. 19/), two bodies with membranous cases and nuclei being 

 attached to a common foot. The foot veiy often consists only of a thin 

 thread, but in other cases it exhibits all the forms which have been described 

 as belonging to the foot of the simple animal. Both bodies are well filled 

 vdih. food. Three, four, or five bodies are frequently seen hanging together 

 in the same manner ; these, however, are by no means in the same plane, 

 but stand out from the foot in various directions. If these animals are ob- 

 tained in considerable numbers, the formation of these colonies by gemmation 

 may easily be observed. The foot is seen gradually to increase in size, and 

 acquire an oval form. A new investing membrane and nucleus are then 

 formed. The offset is always equal to the parent-animal in size. Like the 

 foot of a single animal, the common foot of two or more is, as might be 

 suj)posed, still in a condition to form offsets." This adhesion Schneider 

 prefers to consider an act of gemmation rather than of copulation, and sup- 

 poses its occiuTence among other Ehizopoda. He adds, '' with Perty and 

 Cohn I have also seen a pair of the Arcella vulgaris attached to one another 

 by their openings, of which one (as was obsei'ved by those naturalists) was 

 provided with a white, the other with a yellow shell. The white shell is 

 probably newly formed, and therefore indicates the young specimen produced 

 by gemmation from its companion." 



An aggregation of the animal contents of a Monothalamous shell, such as 

 Cohn noticed in one of the two coherent Difflugice and attributed to an act 

 of conjugation, Schultze has seen in Phizopods, quite independently of that 

 phenomenon. In Lagynis Bcdtica, he states he has frequently seen the con- 

 tents collected into a ball, having a clear speck in the centre, and situated at 

 the posterior end of the shell, without trace of extended fibres ; and he adds, 

 " the origin of this globular mass may be followed in a great number of 

 individuals. The posterior portion of the transparent body of the actively- 

 moving animal gradually becomes darker, owing to the advaneuig develop- 

 ment of numerous molecular and strongly refracting particles. In the midst 

 of this dark portion a clearer spot is always visible, although it cannot be 

 isolated or more intimately examined. By degrees the dark portion en- 

 croaches upon the entire substance of the body, and at last fills up the whole 

 posterior portion of the shell, the body of the animal thus seeming to shrivel 

 up into the ball-like mass described." This process, observed in numerous 



