188 15ALBIAM, ON SEXUAL 



that its floor forms a kind of moveable plateau^ wliicli con- 

 stitutes what has been termed the disc in these species, and 

 is susceptible of raising or lowering itself to a greater or less 

 extent, by means of the contractile stalk on which it is sup- 

 ported. 



If, now, we examine the manner in which copulation takes 

 place among the animals presenting the above type of con- 

 formation, Ave shall remark that they always place themselves 

 parallel to one another, uniting by the depressed region placed 

 in front of the mouth. When, as in some Paramecina, this 

 region occupies a considerable extent of the ventral surface, 

 which is itself deeply excavated, the two individuals receive 

 one another mutually into this hollow, holding together as if 

 embraced by the lateral walls of their bodies. When, on the 

 other hand, the buccal depression exists simply under the 

 form of a tiny pit or straight and superficial groove, their contact 

 takes place only in the space occupied by this pit or groove, 

 and they remain free for the rest of their surface. The exuda- 

 tion of a glutinous substance at the point of contact of the 

 two animals, serves to strengthen their union by fusing them 

 intimately together, so that their separation becomes com- 

 pletely impossible, in spite of the sometimes very strong pulls 

 which, under certain circumstances, they exercise against 

 one another. 



Infusoria of nearly cylindrical or oval form, unite by their 

 oral depressions alone, the rest of the body remaining free. 

 And even when this depression is very short, in consequence 

 of its close proximity to the anterior pole of the animal, the 

 viscid secretion of the parts connects them firmly together, 

 notwithstanding the slight extent of the apposed surfaces. 



If the body be much compressed, as in Amphileptus and 

 Loxophyllum, the mouth appears as a mere slit on its 

 ventral margin, and though, in such cases, the surrounding 

 depression is wanting, yet we find, as before, a row of peculiar 

 cilia running along its site towards the anterior extremity, 

 whose distance from the mouth, therefore, determines the 

 length of the region along Avhich, by simple contact, sexual 

 union takes place. 



Having witnessed in so many species the important share 

 which the oral region seemed to take in copulation, M. 

 Balbiani was led to seek in this region for some mode of 

 communication between the sexual organs of the two con- 

 joined individuals. Long he sought in vain, until, from 

 frequent observation of the peculiar position which the mouth 

 assumes during this act, the two oral orifices being not merely 

 opposite, but closely applied to one another, he was induced 



