NO, 10 CRUSTACEAN METAMORPHOSES — SNODGRASS 47 



in the life history. Species that feed temporarily on the host only 

 during the larval stages may undergo but little structural adaptation. 

 On the other hand, species that are permanently parasitic are likely 

 to go through a high degree of metamorphosis both in the larval and 

 the adult stages. The two species described in the following pages, 

 one belonging to the Gnathiidea, the other to the Epicaridea, may be 

 taken to illustrate the two extremes of parasitic metamorphosis found 

 among the isopods. 



Paragnathia formica (Hesse). — This isopod, parasitic in its larval 

 stage on fishes, gives us a good example of a parasite that undergoes 

 but a minimum of metamorphic adaptation to life on its host. The 

 developmental life history of Paragnathia formica has been amply 

 described by Monod (1926) and the following account with accom- 

 panying illustrations (fig. 18) is taken from Monod's work. 



The adult males and females live together in small burrows exca- 

 vated in semihard mud banks of Stillwater estuaries below the mean 

 level of the ocean. Here the pregnant females in late summer or early 

 fall give birth to active larvae. The newborn larvae leave the burrow, 

 swimming with great speed by movements of the abdomen. Once in 

 the open water they lose no time in attaching themselves to a fish ; 

 most any fish will do. The time between birth and attachment is a 

 period of dispersal, during which the larva takes no food, subsisting 

 on the remains of yolk in its alimentary canal. The larva attaches itself 

 on the fish with its second maxillipeds, and the attack is made at any 

 place that will readily yield blood, such as the membrane between the 

 rays of a fin, the gills, or the mouth. 



The swimming larva (fig. 18 A) is a fully segmented young isopod 

 with large compound eyes and a complete equipment of appendages. 

 In its embryonic development it has been provided in advance with 

 efficient piercing mouth parts and a sucking apparatus. The mouth 

 parts (G) are enclosed in a large conical proboscis projecting forward 

 from the head, formed of the epistome (Epst) above and the first 

 maxillipeds {iMxpd) below. The long, strongly toothed mandibles 

 {Md) are but little movable ; they serve as harpoons to hold the para- 

 site close to the fish while the sharp-pointed, freely movable first 

 maxillae {iMx), supported by the paragnaths {Pgn) beneath them, 

 puncture the integument. The much reduced second maxillae {2Mx) 

 have no recognized function in feeding. 



When the young larva (fig. 18 A) has once established itself on a 

 fish and has begun to feed on the blood of the host, its form changes ; 

 the change is said by Monod to be effected without the intervention 

 of a moult. The thorax lengthens, accompanied by a swelling of the 



