303] XORTH AMERICAN POLYSTOMIDAE—STUNKARD 23 



lu my own material I find that the larval hooklets are invariably 

 present in the bases of the suckers, but of the other larval hooklets, us- 

 ually several are absent and often those present are so arranged that it is 

 difficult to see how they could function in attachment. Those at the an- 

 terior edge of the caudal disc are seldom regularly arranged, and in 

 many cases (Figs. 37 to 43) are in such irregular and unusual positions 

 with reference to each other that the use of one would interfere with the 

 action of the others. 



The great hooks are invariably present in the species in which the 

 caudal disc is cordiform in shape, i. e., where the two anterior suckers are 

 separated by a distance exceeding that between the two posterior suckers. 

 In the species P. alluaudi and P. orbiculare the disc is circular and the 

 great hooks are absent. Usually the cordiform disc is wider and the cir- 

 cular disc is narrower than the body. At first it seemed possible to 

 separate the genus into two subgenera, one in which the disc is circular 

 and the great hooks are absent and another with a cordiform disc and 

 great hooks present, but there seems to be no such clear line of separation. 

 In P. arbiculare a large number of ehitinous spicules are present on the 

 disc, some between the suckers and the others in the central area of the 

 disc. In P. opacum the disc is intermediate in shape ; it is difficult to 

 determine whether it is circular or cordiform, and the great hooks are 

 present altho they are not more than half the size of those in other species 

 (Fig. 40). In P. hassalli the disc at times may be circular and the great 

 hooks are strongly developed (Fig. 31). 



Body Covering. — The body is covered with a non-cellular, unarmed 

 cuticula, which is turned in at the external openings of the various sys- 

 tems. It does not have a uniform appearance but is traversed by lines 

 which extend perpendicular to the surface of the body. 



Musculature. — The musculature consists of the dermo-museular sac, 

 the muscles of the adhesive apparatus, and dorso-ventral strands with 

 much-branched fibers which traverse the body at irregular intervals. The 

 muscles of the body wall consist of an external circular layer, an interme- 

 diate layer of diagonal fibers, and inside the latter, bundles of longitud- 

 inal fibers. In all the species studied, the inner longitudinal fibers are 

 more strongly developed than either of the other layers. Stieda (1870) 

 in P. integerrimum did not distinguish between the two external muscle 

 layers and described only two layers of muscles, an outer layer of an- 

 nular fibers, some of which were not exactly circular and crossed each 

 other, and an inner layer of longitudinal fibers. Zeller (1876) was in 

 error when he described the diagonal fibers as inside the longitudinal 

 layer in P. integerrimum. The arrangement of the muscles of the body 

 wall in Polystoma is the usual condition in the Heterocotylea, and a 



