460 ^- S- NIOKKRSON, 



that ill ihc euibryu uf Äspidoyaster the veiiUal suckor arises as a 

 simple structure which secondarily elongates and becomes separated, 

 first by transverse and later by longitudinal ridges, into the series of 

 sucker-like depressed areas of which the complicated adult organ is made 

 up. lu Stichocotyle there is the equivalent of only one of these series 

 of lines of separation, the transverse; but, as in Aspidogaster , they 

 are formed first at the anterior end. The process differs from that 

 in Aspidogaster in that the subdivisions in Stichocotyle become entirely 

 separate from one another forming distinct suckers, whereas in that 

 form the separation is less complete and they remain connected as 

 parts of a single compound sucker. 



Each of the simple suckers of Stichocotyle (PI. 29, Fig. 4; PI. 31, 

 Fig. 21) has the typical form of such structures, that of a shallow 

 bowl or cup with a circular rim and a thick wall which is sharply 

 separated from the general parenchymatous tissue. The cuticula which 

 covers the surface of the body becomes thinner where it passes over 

 the rim of the sucker and within the depression is not more than 

 one-half the normal thickness. 



The musculature of the sucker is quite simple. It consists of 

 four sets of fibres. Of these the most prominent is that which extends 

 radially from the deep to the superficial surface of the sucker. 'J'liese 

 are well developed fibres and at their ends are generally divideil into 

 a number of strands, by means of which several points of attachment 

 are gained. 



Along both the inner and the outer surfaces of the sucker are 

 delicate muscle fibres which occupy planes approximately parallel with 

 the long axis of the animal and consequently appear in transverse 

 sections as rows of fine dots (PI. 31, Fig. 21). These are continuous 

 with each other at the margins of the cup, where the dots are some- 

 what larger owing to the greater diameter of the fibres in this region. 

 It may be seen in frontal sections that the fibres of this part of the 

 sucker extend in a circular course around the margin of the opening 

 and thus act as a sphincter to diminish the size of the orifice. Scat- 

 tered throughout the substance of the sucker are nuclei which occa- 

 sionally appear to lie between the branched ends of the radial muscle 

 fibres, but in general are irregularly distributed between the fibres. 

 It may be seen in transverse sections of these radial muscle fibres that 

 each one consists of a darker (stained) central core surrounded by a 

 more hyaline cortical portion. 



In all portions of the tissue of the sucker there may also be 



