194 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



anterior part without detaching it. The anterior part, 

 when thus separated from the posterior, retains its vital 

 powers for at least twenty-four hours. At first it swims 

 about vigorously, but after some hours begins to get 

 sluggish in its movements, and then finally dies. The 

 posterior part does not live long when separated from the 

 anterior. The parasites can be kept alive in sea water for 

 upwards of six weeks after removal from the fish. 



External Characters. 



The animal is depressed dorso-ventrally, and is of a 

 more or less oval shape, and distinctly divided into four 

 parts (Plate I., fig. 1). The foremost one of these parts, 

 and usually the largest, is almost circular in outline, and 

 has all the appendages, with the exception of the fourth 

 and fifth pairs of feet, attached to it. This part is known 

 as the cephalo-thorax. 



Viewed from above, this region is seen to be slightly 

 convex ^nd divided into four portions by imperfect 

 sutures. Two of these sutures are longitudinal, and 

 separate the lateral parts of the region from the central. 

 The remaining suture joins the centre of the two longi- 

 tudinal sutures like the cross line of the letter H, dividing 

 the centre of the cephalo-thorax into an anterior and a 

 posterior portion, of which the anterior is the greater. 

 There is also an apparent suture near the frontal margin. 

 The frontal margin is indented, the greatest depth being 

 in the middle line. This indentation to some extent is due 

 to the scar caused by the breaking away of the filament 

 for attachment in the " chalimus " stage. In the centre 

 of the hollow, situated on the ventral surface, is an ovai- 

 shaped opening (b) with a chitiiious fringe. This is 

 evidently a sucker, and represents the remains of a median 

 sucker which is considerably developed during the " chali- 



