AND ALLIED FORMS. 



Fig. 27. 



shape, although the body was capable of great elongation and con- 

 traction, assuming protean forms. The two black eyes were very 

 conspicuous. There are apparently two acetabula, the anterior 

 one of which is described by Diesing (Sys- 

 tema Helminthum, I, p. 300) as the mouth, 

 and it performs the functions of a sucker. 

 There is a small aperture anterior to it. In 

 creeping, the motion is siniilar to that of the 

 looping or geometric caterpillars, the suckers 

 being alternately attached. But in water the 

 animal swims by vibrations, and is so rapid in 

 its elongations, contractions, and gyrations, 

 that the eye cannot follow it. Often it 'takes 

 the form of a round disk, from which the tail 

 projects upward wriggling in the most comical 

 manner. A figure of this little animal, which 

 may be called Histrioaella pomatiopsidis, is 

 subjoined (Fig. 27). The species already known are European, 

 and found upon aquatic snails, such as Planorbis and Viviparm. 

 Besides the lapidaria, there are undoubtedly one or more others 

 of the so-called Amnicol* of North America which belong to 

 this genus as I have defined it ; but with one exception I cannot 

 state with any degree of probability what they are, having seen 

 the animal of lapidaria only. The Amnicola Sayana of Anthony, 

 however, upon which Mr. Gill founded his proposed genus Chilo- 

 cyclus, belongs with little doubt to Pomatiopds ; for the shells are 

 similar in all essential characters, such as the reflected lip,' which 

 is developed only in the adult ; the animal has similar terrestrial 

 habits, and its lingual dentition is of the same general type, as 

 may be understood by a comparison of the accompanying wood- 

 Fig. 28. 



' This is the character ("circular lip reflected"), which was considered 

 by Mr. Grill as the distinctive mark of Chilocyclus. 



