312 ENTOMOLOGY FOR MEDICAL OFFICERS 



grouped in three regions, or the body behind the head may 

 be almost worm-like in simplicity, or the whole animal may 

 be enclosed in a bivalve shell. In any case the posterior end 

 of the body (abdomen) commonly is destitute of appendages 

 and ends in a pair of long filaments, or " caudal fork." All 

 these inconstant forms are conventionally grouped together 

 as Entomostraca. 



Entomostraca. 



In this group, so far as living forms are concerned, four 

 subclasses are included, namely, Branchiopoda, Ostracoda, 

 Copepoda, and Cirripedia. None of the Branchiopods and 

 Ostracods are known to be parasitic, but in the other two 

 subclasses many instances of parasitism have been observed, 

 which, although they may not be of any practical importance, 

 are illustrative and suggestive. Among the Copepods, again, 

 we find the necessary " intermediate host " of the guinea- 

 worm — a formidable subcutaneous parasite of man in certain 

 warm countries. 



Subclass Copepoda. 



The Copepoda (/cwtt;? = oar ; 7roi;9 = foot) are found, some- 

 times in prodigious numbers, in all the waters of the 

 globe, salt and fresh. Many of them are ectoparasites, and 

 not a few are internal parasites, of marine animals : most of 

 the ■ free-living forms are minute, but some of the parasites 

 are of considerable size, one of them — parasitic on a fish — 

 exceeding a foot in length. 



In the free-living Copepods the arrangement of the body 

 and appendages is typified by Cyclops, described below ; but 

 among the parasitic Copepods it is highly diversified and 

 often degraded, and we meet with worm-like forms that have 

 lost their appendages, and with sack-like forms in which the 

 segmentation also has disappeared, curious lobes and excres- 

 cences being sometimes developed on various parts of the 

 body. In certain deformed Copepods the females of which 

 are parasitic on fish, the males are dwarfed and live as para- 

 sites on the females. 



Besides the attached parasites and the internal parasites, 

 there are to be found among the Copepods semi-parasites 



