OF MICRO-ORGANISMS. 69 



varies according to the position of the mouth which 

 in certain groups is lateral and in others terminal, 



The greater number of species have a lateral mouth. 

 To this class belong the Paramecia; these Infusoria, 

 in which the buccal fosse lies at the bottom of a deep 

 excavation made in the ventral face, cover each other 

 over the whole extent of this face, exuding a gluti- 

 nous substance which causes them to adhere in this 

 position; the two mouths then lie exactly upon each 

 other. Copulation lasts from twenty-four to thirty-six 

 hours with the Paramczcium au'relia; it lasts several 

 days (five or six) with the Paramcecium bursaria. 

 Among the Oxytrichinae, the two animals in conjuga- 

 tion blend together at an important part of their per- 

 sons in a very intimate fashion. 



We next arrive to the second group of Infusoria, 

 which show a terminal mouth; of this type we have 

 had a specimen in the Didinium nasutum, the curious 

 hunter Infusory; we may .further mention the Coleps, 

 the Nassula, the Prorodon. The two organisms, in 

 this case, do not embrace laterally, they take a posi- 

 tion end to end, connected by fheir anterior extremi- 

 ties, mouth opposite to mouth; then, little by little, 

 while still joined at the buccal extremity, they shift 

 about until they meet length to length. 



We shall mention particularly, but briefly, the cu- 

 rious phenomena that accompany fecundation among 

 the Vorticels. Even more than in the instances just cited 

 do these phenomena resemble the process of fecun- 

 dation in higher animals, for in this instance fecunda- 

 tion is effected between two differentiated individuals, 

 one of which acts as a male element and the other as 

 a female element. The Vorticels are colonies of In- 

 fusoria in which are found sedentary individuals, 



