210 DR. J. W. MACDONALD ON A NEW GENUS OF TREMATODA, 



There is surely something more than coincidence in this. There may be misgivings 

 as to the position of the Oligoch<eta ; hut perhaps something will soon turn up to recon- 

 cile it. 



One or two additional points of analogy between the Trematoda and the Hirudinei 

 will be seen in the description of the forms which I have now to present to your notice. 



The little Trematode represented in the first figure of the plate was found by me 

 creeping about in the respiratory siphon of a large Melo, or melon-shell, in Shark Bay, 

 "Western Australia. It was between an eighth and a tenth of an inch in length, tallowy 

 white (to use an American expression), and so much resembled a leech as to deceive me 

 at first sight ; for I had already seen white leeches, both parasitic and free, and I did not 

 expect to find a Trematode walking abroad. The fore part of the body was slender, taper- 

 ing, and depressed ; the oral sucker was subterminal, and of the usual cup-like form ; and 

 after a slight constriction, a bulbous pharynx was distinctly seen, from which the oeso- 

 phagus passed backwards for some length, to what would appear to be its bifurcation ; 

 and the appearance of the caeca could be traced to within a short distance of the posterior 

 extremity. But the ventral disk constituted the most striking feature in this little animal, 

 on account of its great size and complex structure. It occupied more than the two 

 posterior thirds of the inferior surface of the body, which was laterally compressed 

 instead of depressed in this locality. The disk, or virtually the foot, might therefore be 

 said to be narrow and elongated in form, and not unlike that of Aspidogaster, though 

 very different in structure. Thus, it presented four rows of alveoli, or deep sucking-pits 

 (fig. 3), the marginal series being the largest, with their long diameter running trans- 

 versely, while that of the intermediate ones was longitudinal. All the contiguous 

 borders of these little pits were so connected by straight bands of muscular fibres as to 

 divide the whole of the intervening area into triangular spaces, the centre of each of 

 which was ocupied by a curious tentaculiform cirrus, capable of complete retraction from 

 the apex, as in the common instance of the finger of a glove (fig. 4). Six rows of these 

 cirri, with thirty in each row, would make up 180, and four rows of alveoli, thirty in 

 each row, 120 ; so that the disk altogether presented a very remarkable appearance. 



The water-vascular system, as in the Trematoda generally, opened posteriorly. The 

 aperture, however, was subterminal and dorsal, or just above and within the hinder 

 border of the disk. The tortuous vessels of this system were seen running forwards on 

 either side of the body as far as the space between the pharyngeal bulb and the oral 

 disk, whence they turned backwards upon themselves, as shown in fig. 5. The genital 

 opening, as in most other cases, was situated in front of the ventral disk (fig. 1, b). 



About the same time I found a large black leech on a species of Myliobatis, presenting 

 at least two characters which were quite new to me — namely, a row of branchial leaflets 

 on each side of the body (figs. 8 & 10), and a caudal sucker alveolated (figs. 11 & 12 

 very much after the manner of the Tremotode already described. 



The head of this creature was depressed, spatulate, and pointed in front (fig. 8). The 

 anterior third of the body was without appendages, constituting a kind of neck, but the 

 rest was furnished on either side with a row of heart-shaped leaflets on short pedicles. 

 Over the base of these, on every fourth segment, was situate a pulsating projection, 



