299] NORTH AMERICAN POLYSTOMIDAE—STUNKARD 19 



ANATOMY AND HISTOLOGY OF THE POLYSTOMIDAE 



The species that have been included in the genus Polystoma show a 

 much wider range of structural variation than is usually present in a 

 natural genus. There are wide differences in the character of digestive 

 and reproductive systems, and variation exists in the type of adhesive 

 apparatus. 



There is wide variation in size ; P. integerrimum, the largest known 

 species measures up to 12 mm. in length, and P. hassalli is only 1.3 to 2 

 mm. in length. The width is one-third to one-fifth of the total length. 

 All the worms that have been included in this genus have a flattened, 

 elongate oval body which at the posterior end bears a large ventral 

 muscular disc or cotylophore. The body is more or less pointed at the 

 anterior end and at the posterior end may or may not have a constriction 

 just before the attachment of the caudal disc. As in all trematodes the 

 shape is subject to considerable variation as the animal elongates and 

 contracts. Locomotion is accomplished by attaching the anterior sucker 

 and then bringing the caudal disc forward; as a result of the terminal 

 attachments and the "looping" method of progression, the dorsal line of 

 the body is more or less arched and the ventral surface is concave. In 

 certain species at the openings of the vaginae on the lateral or ventro- 

 lateral margins of the body, there are prominent swellings, the "Seiten- 

 wiilste ' ' of Zeller. These structures are not present in any of the known 

 North American species. 



Organs of Attachment. — -The caudal disc bears on its ventral face 

 the chief organs of attachment. These consist of suckers and hooks, the 

 former arranged in pairs, three suckers on each side of the median line. 

 The two posterior suckers are close together and those of the middle pair 

 are separated by a considerable distance, while the anterior pair may or 

 may not be near each other. In all previously reported forms except P. 

 alluaudi, the anterior suckers are separated by a considerable distance, 

 giving the disc the shape described by Leidy as eordiform (Fig. 27). In 

 the single specimen of P. alluaudi described by Beauchamp, both the 

 caudual and cephalic suckers are separated, while those of each side are 

 contiguous. In P. orhiculare the anterior suckers are in the same close 

 proximity as the caudal pair, and each sucker of the disc is separated 

 from the two adjacent to it by uniform distances, making a perfect circle 

 of suckers (Fig. 1). In the six species described by the writer these 

 suckers are complicated structures, set more or less deeply in the paren- 

 chyma of the caudal disc. Their structure, character of insertion, mus- 

 cular attachments, and relation to the surrounding tissue indicate that 

 they are protrusible and retractile, and in fact such movements may be 

 observed by watching the live worm. 



