l48 NATURAL HISTORY 



These creatures are of various tints, white, blue, or 

 green : the whirl of ciha is sometimes indented or formed 

 in a spiral line ; in others, interrupted probably at the 

 mouth aperture. Some specimens appear to possess a 

 row of cilia along one edge of their bodies, the utility of 

 which future observations may inform us. They vary 

 greatly in size (from l-20th to 1-lOOth long), which 

 arises either from the quantity of food supplied, or from 

 the state of their growth, as they propagate by division 

 into unequal parts. 



Their increase by transverse division has been recorded 

 by Mr. Trembly, who states that it is first indicated by an 

 oblique stripe where the lips or head of the new animal- 

 cule is to be produced; the commencement of this is ob- 

 served a little below the middle of the body, whence it 

 proceeds about half round the parent : these lips are at 

 first discerned by a slow motion ; they then insensibly 

 approach each other, and close, when a protuberance is 

 formed on the side of the body ; this afterwards becomes 

 the head of the new animalcule, and the two may be 

 plainly distinguished, the superior one being connected 

 with the inferior by its lower extremity, which, by the 

 motion of the cilia, soon becomes detached, and the 

 parent swims away. Sometimes they propagate by lon- 

 gitudinal division, commencing at the head, one polype 

 appearing with two distinct heads, as observed in the 

 pofymorpha. 



360. VoRTicELLA nigra is very similar in structure to 

 the preceding, but the body is shorter and more pointed. 



