The quantitative changes of the primary undifferentiated 

 thorns proceeded along the line of their adaptations to the best attachment 

 to the body of the host and the simultaneous possibility of a relatively 

 easy detachment, i. e, , toward the formation of hooks equipped with 

 musculature and not of constantly acting hook- -anchors, similar, let 

 us say, to the ones on the proboscis of Acanthocephala and of the 

 scolex of tapeworms and so forth. The nature of separate ways of 

 concrete changes will be discussed later during the examination of the 

 systematics of monogenetic trematodes.but for the time being we shall 

 indicate that the morphology of a fully developed edge hook of mono- p. 329 



genetic trematodes- -homologous to the primary chitinous thorns (see 

 page 28 ), shows that it is completely adapted to the fulfillment of the 

 function indicated above. At the same time we must not forget that the 

 edge hook is not an isolated formation: it, together with the adjoining 

 part of the attaching disc and with the corresponding musculature, 

 represents a complex organ apparently equipped also with a special 

 innervation. The basic direction of change of the edge hooks is clear, 

 it is a differentiation in the hook itself (hooked part, nobis) used for 

 the penetration into the tissue of the host, and in the offshoot which 

 serves as a stop during its fastening and, which is most important, 

 during its removal, and finally in the handle, which serves for the 

 attachment of the basic muscles and also as a stem for the supporting 

 of the part of the disc during the attachment of the hook to avoid sliding 

 and to result in a greater contact between the disc of the parasite and 

 the body of the host. Along with this qualitatively new phenomenon 

 appears the phenomenon of the so-called middle hooks which have a 

 different structure (seepage 28 ) and correspondingly also another 

 method of action, although their physiological significance is analogous 

 to the one of edge hooks. Apparently, within the limits of the first way 

 of evolution, the middle hooks as a rule are new formations and are 

 not form.ed at the expense of altered edge hooks (are not merely nnodi- 

 fications of edge hooks, nobis) . The basic line of their development is 

 the increase in strength (not in size, but in function) by way of greater 

 and greater development of both extensions serving for the attachment 

 of muscular bands and also for articulation again with new formations, 

 the connecting plates, which appear as a rule only in the presence of 

 the chitinous hooked armature. The integration along the line of the 

 formation of the system of all middle hooks, middle plates, and 

 muscular and tendon bands is characteristic for middle hooks, whereas 

 each of the edge hooks forms a system which acts independently although 

 generally in concord with each other (however, not always !). 



The coordinated action of the system of the nniddle hooks 

 with a part (predominantly two) of the edge hooks can be considered as 

 the highest degree in integration in the action of the chitinous hook apparatus. 



385 



