8 WRIGHT AND MACALLUM. [Vol. I. 



nation also, the contractile bladders of the excretory system 

 and the receptacula seminis are likely to strike the observer, 

 but the appearance of the various organs in the living animal 

 may be more conveniently considered under the systematic 

 headings which follow. 



The Investing Membrane. 



Zoologists are familiar with the different theories which have 

 been propounded as to the nature of the superficial layer of the 

 skin in the Trematodes. It is unnecessary to detail the history 

 of these here, as they may be found collated in any of the 

 recent German papers bearing on the subject;^ but it may be 

 said summarily, that it has been considered, i , a basement mem- 

 brane, from off which the epithelium has been lost (a view de- 

 fended by Minot in his study of Distomwn crassicolleY ; 2, a 

 cuticula derived from a subjacent epithelium in which the cellular 

 elements are more or less distinct (Leuckart olim, Sommer, 

 Taschenberg, etc.) ; and, 3, a syncytium resting directly on the 

 circular muscles, from which the nuclei have generally disap- 

 peared (Ziegler, Looss, etc.). The developmental evidence sup- 

 plied by Leuckart and Schwarze ^ appears to us to establish the 

 third view; but we are unable to furnish any account of the 

 origin of the investing membrane in Sphyranura, which is, how- \^ 

 ever, in the adult certainly homogeneous, destitute of nuclei, 

 and rests directly without the intervention of a subcuticular 

 layer on the circular muscular coat. 



The investing membrane in Sphyranura is very elastic, and 

 thus measurements of its thickness vary with the condition of 

 contraction or extension of the body. In a carefully fixed 

 specimen its thickness is about 4.2 f.L, except where, as in the 

 caudal suckers, a local thickening occurs. In specimens fixed 

 during violent contraction the investing membrane is thrown 

 into transverse wrinkles, a condition which can also be seen dur- 

 ing life, and is no doubt due to the arrangement of the circular 

 muscular fibres. 



If a fresh specimen be examined under a high power there will 

 be seen projecting to 12 or 13 ^u beyond the free surface a great 



'Very completely in that of Looss Zeit. vviss. Zool., XLI., 391. 

 •Mem. Bost. Soc. Nat. Hist., Vol. III., 2. 

 'Zeit. wiss. Zool., XLIII., 41. 



