The Evolution of the Excretory System in Certain Groups of the Furcocercous

Within the last few years numerous workers have added yery materially to the sum total of our knowledge of the detailed structure of a· number of furcocercous cercariae and we are now in a position to attempt a systematic grouping of the various species based on the c}ln.racteristics of their excretory systems. In 1922 I attempted a classification of the furcocercous cercariae and created a number of groups in accordance with the number of flamecells in the body and tail-stem. Since the publication of this paper, Miller (1926 and 1927) has attempted a further subdivision of certain of these groups and has created others to accommodate new species. This author now sub-divides the furcocercous cercariae into two main divisions according as a pharynx is present or a b~ ent. These two ma.in divisions, Pharyngeal and Apharyngeal, are again split up into (a) those that possess long furcal rami, these being longer than one-half the length of the tail-stem, and this group he terms the Longifurcate group; and (b) those tha.t have short furcal rami, the Brevifurcate group, the furcal rami in these instances being shorter than one-half the length of the tail-stem. This latter group, however, in the light of more recent discoveries requires modification, for, as Miller himself has shown i~ a recent paper (vide i~lra, p. 363), certain cercariae, that in all ~thel' characters agree with and belong to the Brevifurcate group, actually possess furcal rami that are slightly longer than half the taiI-etem. Finally, a further subdivision of these groups is based on the presence or absence of an acetabulum; we thus have Monostome. and Distome groups, but it must be borne in mind that the absence of an acetabulum in the larval sta.ge may be due merely to a retarded development of this organ and does not necessarily imply that such a form will develope into a Monostome adult. "\\T e can thus recognise the following main divisions :-