644 PROCEEDINGS OF THE XATTONAL MUSEUM. vol. xxv. 



If the fish has been reiiioved from the water the parasite usuall}^ 

 remains within the branchial cavity since this retains moisture long^er 

 than any other portion of the external surface. It is no uncommon 

 thing to find them alive there several hours after the fish's death. 



Upon removal to an aquarium the Argulids are found to have retained 

 their locomotor ability to a greater degree than any other group of 

 the parasitic copepods. 



And there is no distinction of sex in this, for males and females 

 alike swim about with as much freedom and as great rapidity as any 

 of the so-called free copepods. And their sucker feet enable them to 

 rest by attaching themselves to the sides of the aquarium, to stones, 

 algae, or any other convenient surface, instead of by balancing after 

 the manner of Cyclopia and allied forms. In swimming, the four pairs 

 of legs are used as the propelling agents, and are provided with a 

 fi'inge of long plumose seta? for that purpose. 



The abdomen is elevated at an angle of about 45 degrees with the 

 plane of the bodj' and seems to serve somewhat as a rudder, but the 

 most of the steering is accomplished by a fiexion of the thorax on 

 the head carapace. The result is an easy gliding motion, wholly des- 

 titute of the jerkiness so characteristic of free copepods, and more 

 resembling that of Artemia and Branc/tipus. They usuall}" move 

 with sharp turns in nearly every direction, often making a complete 

 summersault, or turning upward and scuttling along back downward 

 on the under side of the surface film of the water after the manner of 

 some snails. 



But though their ordinary motion is slow and easy, the}^ can dart 

 about with considerable rapidity upon occasion. 



The length of the plumose setae on the swimming legs seems to 

 determine in great measure the rapidity of movement. In some spe- 

 cies {laticauda, ^oer si color ^ etc.) the setae are long and stout, while in 

 others {niger, alosae., etc.) the}^ are short and weak. It follows that the 

 former species are capable of much more rapid and energetic motion 

 than the latter. 



In an aquarium these Argulids seldom exhibit the disagreeable pro- 

 pensity' shown b}' the Caligids of crawling up as far as possible on the 

 sides of the aquarium above the surface of the water and remaining 

 there until dead and dried up. 



On the contrary, they are easil}' kept in confinement and make 

 docile and highly interesting laboratory material. 



As a natural consequence of its freedom of motion, an Argulus is not 

 as closely confined to a single species of fish as are the other copepod 

 parasites. This fact is very apparent from an examination of the list 

 of hosts following the description of each species (p. 704). 



It is to be remembered in this connection that our knowledge 

 of these American forms is as j^et extremely meager. When some 

 of them come to be known as well as the European A. follaeem^ the 



