16 



the entrance to the oesophagus. There are also an upper and an 

 under lip. The whole structure is controlled by strong chitinous 

 tendons connected with the muscles of the carapace. At the upper 

 end of the siphon we find a sheath containing a long telescopic 

 proboscis, called a poison-sting by Claus. It is a beautifully slender 

 and exquisitely pointed weapon, capable of movement in any direc- 

 tion ; it may be extended or retracted very rapidly. Its function is 

 difficult to determine. Claus thinks that by reason of its irritating 

 effect on the skin of the fish that it induces inflammation and a flow 

 of blood to the punctured part. As will presently be shown, the 

 blood of the fish forms the food of the parasite. 



The thorax has four slightly defined segments, some say five, best 

 seen in the larval state. These correspond to the four pairs of 

 swimming-feet which are attached. It commences at the base of 

 the mouth, and contains the large and small stomachs, and at its 

 lower extremity the dorsal heart, where it is terminated by the 

 tail-plate. In the gravid female the ovarium occupies the whole 

 of its ventral surface. 



Describing the feet in their proper order, one must commence 

 with the pair of foot-jaws (so called). These arise from a little below 

 the mouth on both sides, and lie in the middle of the carapace. 

 Each consists of five cylindrical joints terminated by two claws and 

 a fleshy, claw-like pad. Numerous spines cover the surface of each 

 joint. To each basal joint are attached three strong chitinous 

 teeth, whilst four similar teeth unattached to the foot-jaws lie 

 between their bases. The foot-jaws have an independent move- 

 ment, and in this respect differ from the four pairs of swimming- 

 feet. Their use is apparently to assist the A?-giihis in attaching 

 itself to the fish, and in locomotion. They undergo little change 

 in structure from the earliest stage to the adult. 



The swimming-feet are most important organs (Fig. lo). There 

 are four pairs placed on either side of the thorax. From the 

 basal joint of each foot proceed two branches of about equal 

 length, bearing pectinate hairs. These latter have a secondary 

 structure of beautiful plumose cilia, which may be stained with 

 nitrate of silver to render them more visible. The first and second 

 pairs in both male and female bear short branches, which arise 

 from the basal joint near the fork of the two main branches on 

 the dorsal side, and curve backwards. Some authorities state that 

 these recurved branches occur also on the third pair, but the most 

 careful observation on my part has failed to reveal such. In the 

 case of the male, as will be afterwards noticed, the organs of 

 generation are situated on the third and fourth pairs of swimming- 

 feet. The corresponding feet in the female have no special organs. 



The suckers are prominent organs, lying a little below the paired 

 eyes, but farther apart. They are the structures referred to in my 

 notes on the fourth larval stage, where they replaced the first 

 pair of foot-jaws. They are fleshy, cylindrical appendages, cartila- 



