22 ILLIXOIS BIOLOGICAL MOXOGRAPHS [302 



A cuticular framework similar to that present in Polystoma was 

 described by Wright and Macallum for the suckers of Sphi/ranura osleri. 

 They say: "As the wall of the sucker is itself destitute of contractility, 

 another arrangement exists for modifying the shape of the cavity. Its 

 walls is really divided into three concentric zones, which by special ex- 

 trinsic muscles can be worked independently. The two circular lines 

 which separate these zones, are marked by an infolding of the investing 

 membrane, which forms a sort of joint, permitting an independent move- 

 ment of the zones." 



The collection of Professor Ward contains a single series of sections 

 of P. intcgcrrimvm from Germany, and in this specimen the type of 

 skeletal structure just described is absent. Figure 45 shows the charac- 

 ter of the suckers in this form. The caudal disc typically bears eighteen 

 hooks. Sixteen are similar in size and shape, arranged as follows : six 

 in a row between the anterior suckei's, one situated inside each sucker 

 at the base, and four between the two posterior suckers. In addition to 

 these hooks there is a pair of great hooks, several times the size of the 

 small hooks, between the two posterior suckers. The shape of these hooks 

 and their arrangement are shown in Figures 37 to 43. In many cases 

 there is only one pair of the small hooks between the caudal suckers ; in 

 such eases in addition to the great hooks there is a third pair, similar in 

 shape to the great hooks and intermediate in size between the great and 

 small hooks. 



The sixteen small hooks are present on the caiulal disc of the larva 

 before the suckers are formed and are called larval booklets by Wille- 

 moes-Suhm (1872), but Zeller (1876) says: "Die sechszehn kleinen 

 Hakchen mit iliren Oesen, welche die Haftseheibe angehoren und welche 

 bei der Polystomum larva so ausscrordentlich deutlich zu erkennen ist, 

 sind nieht, wie Willemoes-Suhm meint, nur 'Larvalorgane'. Sie werdeu 

 nicht abgeworfen, sondern sind wie ich auf das bestimmeste wiederholen 

 muss, bei der erwachsenen Thiere noch sammtlieh vorhanden, sehr 

 beweglich und gewiss nicht ohne Bedeutung fiir ein festeres Anheften." 

 Jolinston (1912) in the description of P. huUicnsc says: "Four larval 

 booklets are present in a row- on the ventral surface near the posterior 

 edge of the disc or cotylophore. I have been able to find no trace either 

 in the living worms or the fixed material, of the larval booklets which 

 P. intcgerrimum and other species bear near the anterior edge of the 

 disc. There is a small anchor shaped hook in the base of each sucker. 

 All these hooks either disappear as the animal increases with age, or very 

 readily become detached. In only one out of sixteen specimens have the 

 whole four posterior booklets been present ; and in only two others were 

 any booklets at all to be seen. In all other specimens no liooklets could 

 be made out." 



