ENTOMOSTRACA. 



27 



more permanent ictoparasites ; still others are endoparasites, living imbedded in the 

 body-cavity or tissues of the host, and living on his blood and lymph. Such forms 

 are often distorted in growth far from the ordinary crustacean type. 

 Nothing can remind one less of a crab, or the nearest allies of the 

 Siphonostomata, the Copepoda, than the misshapen forms of the 

 Lernceocera or Hceinobaphes. Other forms are less widely sepa- 

 rated from the usual type of Crustacea, and remind one of dis- 

 torted and ill-shaped monstrosities ; while the temporary parasites 

 are possessed of well-developed appendages, and in all respects 

 conform to the usual type. We have, then, all grades of struc- 

 ture, all the modifications which can be induced by a parasitism here 

 displayed on a scale, and with a completeness which no other 

 group in the animal kingdom affords. The parasitic insects are 

 too little altered in structure, the parasitic worms are all too pro- 

 foundly modified to give the range of this order. Again, develop- 

 ment here shows the connection of the various forms. However 

 unlike the crustacean form the adult may be, the larva always 

 belongs to the nauplius type, and thus indicates the real affinities 

 of its parents. 



The hosts of these parasites are as various as their forms. 

 Fishes of all kinds, whales, mollusks, worms, and ascidians are all 

 provided with these guests. As on land almost every species of 

 bird or mammal has its own parasitic insects, so in the water 

 almost every species of fish or larger invertebrate has its para- 

 sitic Siphonostomata. Numerous varieties of parasites often infest 

 a single host. The haddock, for instance, has more than a dozen 

 kinds of parasites, some attached to the skin, others to the fins, 

 others still to the gills, or to the skin of the mouth, or imbedded 

 in the muscles. Other fishes have nearly as many forms ; and 

 careful study would, no doubt, vastly extend the number for all 

 sorts of fishes. They attach themselves to the skin of worms or Crustacea, -- to the 

 gills of the latter group, burrow in the flesh of sea-snails, and suck the blood of the 

 cuttle-fishes. Some forms live in the gill-sac of ascidians, either as commensals or as 

 true parasites ; and, finally, star-fishes, jelly-fishes, and corals are not exempt from these 

 omnipresent pests. 



From these parasitic habits these forms have acquired the vulgar name of "fish- 

 lice," and the order itself is called, by some authors, Epizoa, for obvious reasons. 

 Though closely allied to the preceding order, and, in fact, by some authors united with 

 it, the Siphonostomata seem to be sufficiently distinct to be entitled to a distinct order. 

 Forms like Caligus closely resemble the Copepods ; but the more typical species of 

 these parasites have undergone a most marvellous retrograde metamorphosis, losing 

 almost everything which could be used as proof of their proper position in any scheme 

 of classification were the adults, and especially the females, studied, the larval stages 

 and the males being ignored. 



We will begin the series of parasites with A/rgulus, a temporary parasite, living 

 on the blood-plasma of fishes drawn from the blood-vessels of the gills, but capable of 

 leaving one host for another, and of maintaining an independent life for a considerable 

 time, living apart from the host as long as a week or ten days after it has filled itself 



FIG. 33 Dinematoura 

 ferox. 



