138 Descriptions oj Frejparations. 



existing between tlie Leech and the Tapeworm, must not be taken 

 as justifying- the views of writers who would class Hirudineae with 

 the Platyelminthes". 



<= The class Platyelminthes is here taken as comprehendini? three orders — the 

 Cestodes, Trematodes, and Turbellarians ; and it is with the Trematodes that the 

 Leeches have been supposed to be so closely allied as to justify the removal of them 

 from the class Annulata, which comprehends the Polychaeta, Gephyrei, and Oli- 

 gochaeta. The principal reasons for this dissociation are those furnished by the 

 absence of external appendiculate organs such as locomotor setae or gills, and the 

 presence of suckers in both Trematodes and Leeches ; by the sacculate character of 

 the digestive tract ; by the absence of a body cavity ; and by the structure of its 

 skin. In answer it is to be said that the absence or presence of such organs as 

 setae or gills is not to be considered as of such consequence as the similarity or 

 dissimilarity of such systems as the reproductive or nervous ; and that even if only 

 external characters are to be compared together, the definite segmentation of the 

 Hirudineae differentiates them very sharply from the Trematodes, which are not 

 even annulated. With reference to the similarity which the dendritic digestive tract 

 of certain Trematodes presents to the diverticulate tube of the Hirudineae, it must 

 be borne in mind that amongst the Polychaeta, forms with more complexly diver- 

 ticulate intestinal tubes, Aphroditea, are to be found than amongst the Leeches ; 

 whilst the presence of an anal sucker in other members of the same order, Leucodore 

 and Clymene, furnishes a similar answer to the argument for classing the Hirudineae 

 with the Platyelminthes which is based upon their common possession of these 

 organs of adhesion. Another answer is furnished by the fact that the construction 

 of the suckers is, as L-suckart has pointed out, by no means identical in the two 

 classes under comparison ; and that the possession of suckers is a point of physio- 

 logical rather than of morphological importance, is even more clearly shown by their 

 existence on the cai:dal extremity of the free, and the ventral surface of the para- 

 sitic Nematoids, which belong to a class very distinct fi-om both Annelids and 

 Platyelminthes. Neither are the Hirudineae truly 'parenchymatous' or ' sterel- 

 minthous ' Vermes in the same sense as the Trematodes. For Leuckart has shown 

 that in all Hirudineae more or less of a perivisceral cavity remains, after the full 

 development of the large digestive tract, and of the 'dorso- ventral' muscles which 

 encroach so much upon it. In Branchiobdella there exists a large perivisceral 

 cavity, as well as a system of vessels, which appear to be homologous with the 

 so-called 'pseud-haemal' vessels of the common Leech and other Annelids, though 

 at the same time they are continuous with, and must be supposed to represent a 

 part of the perivisceral cavity. And by consequence, therefore, the pseud-haemal 

 vessels of other Hirudineae nmst be taken to represent the remnant of the peri- 

 visceral cavity, when such a space appears to have become obliterated. So that 

 the true way of expressing the facts would be to say, not that the Hirudineae re- 

 semble the Trematodes in not possessing a perivisceral cavity, but that they differ 

 from them in possessing a system of vessels which, as being continuous with a peri- 

 visceral cavity in Branchiobdella, may be regarded as actually being in the species 

 mentioned, and as representing in other species a part of a perivisceral cavity. To 

 this interpretation the fact that in certain Polychaetous Annelids, Glyccra and 

 Phoronis Uippocrepia, the pseud-haemal vessels have beeu observed to contain true 



