Ill PODOPLEA AMPIIARTHRANDRIA 63 



Fam. 3. Peltiidae/ — This is an interesting family, allied to 

 the Harpacticidae, and includes species with flattened bodies 

 somewhat resembling Isopods, and a similar habit of rolling 

 tliemselves up into balls. No parasitic forms are known, though 

 Sunaristes paguri on the French and Scottisli coasts is said to 

 live commensally with hermit-crabs. 



We have now enumerated the chief families of free-living 

 Copepods ; the rest are either true parasites or else spend a part 

 of their lives as such. A number of the semiparasitic and 

 parasitic Copepods can be placed in the tribe Ampharthrandria 

 owing to the characters of their antennae ; but it must be 

 remembered that many parasitic forms have given up using 

 the antennae as clasping organs ; however, the sexual differences 

 in the antennae, and the fact that many of the species which 

 have lost the prehensile antennae in the male have near relations 

 which preserve it, enable us to proceed with some certainty. 

 The adoption of this classification necessitates our separating 

 many families which superficially may seem to resemble one 

 another, e.g. the semiparasitic families Lichomolgidae and Ascidi- 

 colidae, and the Dichelestiidae from the other fish-parasites ; it 

 also necessitates our treating the presence of a sucking mouth as 

 of secondary importance. This characteristic must certainly, how- 

 ever, have been acquired more than once in the history of the 

 Copepods, for instance in the Asterocheridae and in tlie fish- 

 parasites, while it sometimes happens that genera belonging to 

 a typically Siphonostomatous group possess a gnathostome, or 

 biting mouth, e.g. Ratania among the Asterocheridae. Again, it 

 is impossible even if we use the character of the mouth as a 

 criterion to place together all the true parasites on fishes in one 

 natural group, because the Bomolochidae and Chondracanthidae, 

 which are otherwise closely similar to the rest of the fisli-para- 

 sites, possess no siphon. It seems plain, therefore, that the 

 parasitic habit has been acquired several times separately l)y 

 diverging stocks of free-swimming Copepods, and that it has 

 resulted in the formation of convergent structures. 



Fam. 4. Monstrillidae.' — These are closely related to the 



Harpacticidae. The members of this curious family are parasitic 



during larval life and actively free-swimming when adult. There 



^ Claus, Copepodenstudie7i, 1. Heft, Vienna, 1889. 

 - Malaquin, Arch. Zool. Ea"p. (3), ix., 1901, p. 81. 



