Many-headed Bladdtr-ivorm. 139 



For a further accounts, with figures of the metamorphoses, of the 

 Taeniadae, see Description of the semi-diagrammatic figures, 

 1—6, in plate xii. infra. See also Cobljold^s Entozoa, 1864, 

 pj). 104 seqq. et j^assim ; Leuckart's Mensehlichen Parasiten, 

 pp. 181-230; and p. 251 for the formation of the proglottides 

 hj the develojjment of annular constrictions in the vermiform 



corpusculated blood, appears to lend considerable probability. On tlie other hand, 

 the Hirudineae do to a certain extent resemble the Trematodes, in possessing a 

 muscular system which is more complex, and more closely connected with the 

 glandular and epithelial elements of the integument, than is usually the case in other 

 Vennes. It may be suggested however that the changed relations of the contractile 

 and other elements of the integument in the Leeches, may be correlated with the 

 other changes which are ordinarily produced in subordination to the special habit of 

 parasitism ; and which, in this sub-kingdom, appear to entail the loss of the setae, 

 the gills, and the cilia. And the development of an additional transversely crossing 

 set of muscles in addition to the external circularly arranged and the internally 

 longitudinally arranged muscles of other Vermes, seems similarly referrible to com- 

 munity of habits, and not to any morphological affinity subsisting between the 

 Trematodes and Leeches. For a similar layer of muscles has been observed by 

 Dr. Charlton Bastian to exist in the Neniatoids (see Phil. Trans., 1866, p. 564) ; 

 and as in all three orders alike, the special need for some such arrangements, for 

 propelling and otherwise acting upon the contents of a digestive tube, altogether 

 or nearly devoid of muscular fibres, may be considered to account for its presence, 

 it cannot be held to furnish a good basis for classification. 



On the whole, the Hirudineae appear to be rather Annelids degraded by the special 

 habit of parasitism, than to be intermediate forms in a series which should represent 

 the various stnges in progression upwards from the lower Platyelminthous Vermes 

 to the highly organized Annelids. The characters which appear to approximate 

 them to the Platyelminthes, relate mainly to such external points as the shape of the 

 body, and the modifications of the tegumentary system ; and whilst these characters 

 are probably to be re^arded as explicable by reference to a community of habit, and 

 therefore as devoid of classificatory value, those which connect the Hirudineae with 

 the Annelids possess a real morphological importance. Among these we m:iy 

 mention the segmentation of the body not merely in the way of external annulation, 

 b\it in that of internal division into more or less completely separated compartments 

 by the development of dissepiments ; the possession of a chain of nerve ganglia, and 

 in most cases of a similarly multiple series of segmental organs ; and the presence of 

 the so-called 'pseud-haemal' system. And it should be further noted, that whilst in 

 all these points the organizations of the Hirudineae and the setigerous Annelids 

 resemble each other, and differ from those of the Platyelminthes in the direction of 

 greater complexity and perfection, they still further resemble each other but in the 

 reverse direction, and as presenting lesser complexity and importance, when we come 

 to compare their reproductive systems, which never possess either the high degree of 

 morphological differentiation, or the actual bulk relatively to the rest of the body, 

 which distinguishes the generative organs of the Platyelminthes. See pi. viii., ix., 

 and pi. xii., figs. 2 and 4 with descriptions, infra. 



