148 s- <^o™- 



means of which the worm can attach itself to foreign bodies sufficiently 

 firmly as to drag, its body after it. We are therefore compelled to 

 admit the suctorial action of both the anterior and the posterior suckers.- 

 This action I shall now proceed to explain. 



The so-called chitinous rods and pieces that compose the sup- 

 porting frame-work of the sackers are, as I have already stated, not 

 formed by true chitin, but are soluble in caustic potash, and in some 

 species stain more or less, and do not seem to be rigid. On the contrary, 

 I believe they are easily flexible but elastic, so that when distorted 

 by external force they constantly tend to resume their original form. 

 Iq fact, if a living worm be observed under the microscope the suckers 

 are seen to constantly change their form; and this seems to be im- 

 possible if the so-called chitinous. frame- work be perfectly rigid. I also 

 believe that the prismatic fibres which compose the wall of the suckers 

 are likewise elastic; this seems to be very probable from their great 

 (doubly?) refractive power and fi*om their other optical characters. 

 The image, then, I form of the suckers from the physiological 

 point of view is that of a bag. (hemispherical or rectangular) with 

 a thick, elastic wall which constantly tends to be flattend out, but 

 which is kept in proper shape by an extei'nal force, that of the 

 muscular fibres that are attached to the bottom of the suckers. If 

 these muscular fibres relax, the wall of the sucker becomes flattened 

 out by virtue of its elasticity and applied to its substratuin, the 

 body-surface of the host; if now the muscular fibres contract, the 

 sucker assumes the form of a bag, and thus a vacuum tends to 

 form within, giving rise to a suctorial action. In favour of this 

 view it may be mentioned in addition that in many sections of the 

 suckers the prismatic fibres are seen to be much pressed against one 

 another at the inner side of the suckers near the point where the wall 

 is folded on itself— the part where the fibres must necessarily be 



