TETKAEHYNOHIDiE. 133 



TetrarhynchidoB. — The members of this family are easily 

 recognized by the possession of four armed retractile proboscides 

 attached to the head. The armature consists of several successive 

 rows of sharply pointed recurved hooks, frequently amounting 

 to several thousands. The head itself is usually more or less 

 bilobed, each half supporting either one bipartite bothrium, or 

 else two separate fossae. These cavities are also frequently 

 supported on four petaloid appendages, which vary much in shape 

 in the different species, and also, in the same individual, according 

 to the degree of contraction of the part. The head and neck are 

 continuous, and usually about the same breadth as the body, the 

 latter being sometimes even narrower than either the head or 

 neck. The body is depressed, filiform, distinctly segmented, and 

 usually of great length in the mature state ; the reproductive 

 orifices being situated at the lateral margin of the joints in an 

 irregularly alternate manner. 



Structure, Habits, and Development. — The various species of 

 this family dwell only in the bodies of marine vertebrata, the 

 mature forms being exclusively confined to the intestinal canal of 

 sharks and rays, whilst the larv^ inhabit various fishes on which 

 the plagiostomi feed. The immature forms also occur in cuttle- 

 fishes, but they are most abundant in such fishes as the cod, 

 haddock, turbot, flounder, sole, gurnard, mackerel, mullet, and 

 conger-eel. One well-marked larva is an almost invariable guest 

 in the muscles and viscera of the great sun-fish. If a number of 

 these larval tetrarhynchs be taken indiscriminately from the above- 

 named hosts, and be compared with one another, it is surprising 

 how greatly the forms will be found to differ. This difference has 

 reference not so much as respects their size and general appearance, 

 but rather as regards the form of their proboscides and the relative 

 number and disposition of the hooks which they carry. Into details 

 respecting these variations it is needless to enter, especially as the 

 selection of any one form will sufl&ce to convey an accurate concep- 

 tion of the leading structural peculiarities belonging to the group. 



