NO. 10 CRUSTACEAN METAMORPHOSES — SNODGRASS 29 



host, since the old filament remained with the discarded copepodid 

 skin. Heegaard (1947) gives an interesting account of how the 

 young chaHmus with the pointed frontal lobe of its head bores a hole 

 in a fin ray of the host. Into the wound thus formed is injected the 

 secretion from the head gland, which hardens and holds fast, while 

 the chalimus backs away and draws it out into a filament that secures 

 the parasite to the host, but still allows it to move about on its tether. 

 According to Heegaard each of the four succeeding chalimus stages 

 reattaches itself in the same manner. The chalimus (C) was given its 

 name because when first discovered it was thought to be the adult of 

 an unknown species. Since the chalimus stages progressively de- 

 velop from the second copepodid to the adult (D), they evidently 

 represent the later copepodid stages of free-living copepods. 



The adults of C aligns cur His (fig. 10 D) have pretty much the 

 form and structure of an ordinary copepod, but, having no attachment 

 to the host, both the males and the females are free to swim away. 

 The egg-carrying female of another species with similar habits is 

 shown at E of the figure. Since these copepods are dependent on a 

 host for food in the adult stage, they retain their parasitic habits and 

 are generally found crawling and feeding on the host, though they 

 have not become specially modified in structure for a life of parasitism. 

 This condition of dependence on a host, however, Wilson (191 5) 

 points out, constitutes the first step toward adult degeneration. If 

 the adult parasite finds it advantageous to remain on the host, organs 

 of locomotion become unnecessary, and in the end all that is needed 

 are organs of nutrition and reproduction. The species shown at F, 

 parasitic in an ascidian, still retains its appendages and a segmented 

 abdomen, but the thorax has taken on a strange shape. The female 

 at G, however, a permanent parasite on the gills of a fish, has de- 

 generated from the copepod structure almost to the limit of simpli- 

 fication. Yet, as already noted, "degeneration" is merely adaptation 

 by the elimination of unnecessary organs. 



An example of an intermediate degree of degenerative simplification 

 is seen in the lernaeopodid fish parasite Achtheres amhloplitis (fig. 

 11) described by Wilson (1911). In this copepod, Wilson says, the 

 naupliar and metanaupliar stages are completed in the egg, and the 

 larva hatches as a copepodid (A). During the egg stage the head 

 gland produces a filament, which is still coiled in the head of the 

 emerging copepodid (A, /). The young larva has two pairs of feath- 

 ery swimming legs, and its maxillipeds (Mxpd) are armed with strong 

 hooks. It swims actively in search of a host, which must be a fish 

 of the surface-swimming Centrarchidae. That the young copepod 



