108 REPORT — 1855. 



1856; his special object being to show the very wide range of variation that pre- 

 sents itself in this type, within the limits of a single species, in illustration of his 

 evening discourse on the general question, "What is a species? " 



Description of a Neiv Species of Tremotode Worm (Fasciola gigantica). By 



T. Spencer Cobboi-d, M.D., Assistant Conservator of the Anatomical 



Museum, University of Edinburgh. 



In respect to this Entozoon, Dr. Cobbold observed as follows : — "The trematode 

 now before the Association, designated Fasciola gigantica,\anes in length from an inch 

 and a half to nearly three inches, most of the specimens being about two inches ; 

 their breadth averages three lines, some attaining the third of an inch. The general 

 form of the body is elongated, and rounded at the caudal extremity, in which latter 

 feature it differs very markedly from F. hepatica. The larger or more fully developed 

 individuals present slight irregularities or crenations of the lateral margins near the 

 neck ; a character, however, by'no means constant. The borders are more attenu- 

 ated than in the common species, and the substance of the body is thinner. The 

 anterior extremity is prolonged forward about two lines, and terminates in a sucker 

 half a line in diameter. There is no evident distinction between what has been 

 termed head and neck, but the part to which the latter title is assigned is very pro- 

 minent on the dorsal surface, from the distended condition of the oviducts and semi- 

 nal reservoir lying immediately beneath. 



" The digestive apparatus commences by a short oesophagus proceeding downward 

 from the base of the oval sucker; while in the neck it divides into two slightly 

 diverging trunks which pass on either side of the ventral sucker, again approximate, 

 and are continued to the tail. On their passage down, the two principal trunks lie 

 almost parallel, near the mesial line of the body ; they give off eight or ten secondary 

 branches, which proceed to the lateral margins, and end in blind caeca ; small 

 twigs also proceed from the main tubes inwards, but they do not extend beyond the 

 middle line, and present very few subdivisions. The ramifying systems of digestive 

 caeca in each lateral segment of the animal are not absolutely symmetrical, neither is 

 there uniformity in respect of number ; they preserve, however, a general resemblance 

 both in the degree of subdivision and in the direction which the secondary trunks 

 assume. The downward direction of the branches, and the angle of divergence re- 

 sulting from such a disposition of parts, form a striking contrast to the arrangement 

 of that system of canals situated nearer the dorsal aspect of the body, and usually 

 regarded as the circulatory apparatus. These vessels are represented in F, gigantica 

 by a single median trunk, from which numerous primary branches pass obliquely 

 upward to the sides. 



" We may here remark, that considerable dispute has arisen among helmintholo- 

 gists, as to the propriety of regarding this series of canals as vascular ; some have even 

 expressed doubts as to the presence of any true organs of circulation in the trematode 

 •worms, and the distinguished authority Van Beneden holds this opinion. Those 

 who regard the superficial set of tubes in the light of an excretory or secreting gland, 

 ground their view on the circumstance of a supposed caudal opening, through which 

 matters thrown into the median vessel frequently pass. M. Blanchard has shown 

 the aperture in question to result from over- distension of the canal, which readily 

 gives way at this, its weakest point ; our own attempts to inject have confirmed this 

 observation. 



" Accepting M. Blanchard's explanation as correct, we have to state further, in re- 

 gard to these vessels, that they exhibit less regularity of distribution than obtains in 

 the branching tubes of the alimentary system, and they inosculate freely from one 

 end of the body to the other. Irrespective of these distinguishing marks, there is a 

 disparity of calibre between the two sets of tubes, and all their peculiarities taken 

 together strongly convince us of their true vascular nature. 



" The external spiral appendages, with minute orifices of the reproductive organs, 

 occupy the same relative position as in F. hepatica, i, e. lying directly in front of 

 the second or great ventral sucker. In reference to these structures — the nervous 

 system and other special parts — it is unnecessary to give additional particulars ; 

 their characters resembling in all respects those seen in the typical species, and 

 which are now so fully understood. 



