46 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, I3I 



surface. Thompsonia, unlike Sacculhia, appears to do no specific 

 damage to its host, so that it can continue its parasitic Hfe and indefi- 

 nitely repeat its reproductive processes. The inoculation of the host 

 by the free-swimming cypris has not been observed. 



The Thompsonia-mitsitd crab presents one of the most curious 

 anomalies in the whole realm of nature. Here are two crustaceans, 

 one inside the other, the crab a highly developed arthropod, the para- 

 site, a crustacean relative of the crab, spread out inside the latter in 

 the form of a network of filaments. Both host and parasite are adult 

 animals, each being the reproductive stage of its species. Progressive 

 and regressive evolution could hardly reach a greater degree of 

 divergence. 



Thompsonia is known to be a crustacean because it produces free- 

 swimming cypris larvae, it is known to be a rhizocephalan because of 

 its likeness to Sacculina, and Sacciilina is known to be a cirriped 

 because of the character of its nauplius. The barnacles and the 

 rhizocephalans have in common the habit of attaching themselves to 

 a support by the antennules in the cypris stage. From this point on 

 they widely diverge. It would be highly interesting to know how the 

 Sacculina larva learned to attach itself at the base of a hair on a crab, 

 how it acquired the urge to get into the crab, and how it ever de- 

 veloped a self-reducing method for doing it. Halfway measures 

 would be useless. Clearly there are problems in evolution for which 

 natural selection does not of!^er a ready solution. 



ISOPODA 



Most of the Malacostraca are too large to be parasites. The ma- 

 jority are predatory, and few of them exhibit any considerable degree 

 of metamorphosis. Most of them, moreover, hatch at a later period 

 of development than do the Entomostraca, and some of them are al- 

 most completely epimorphic. A prominent exception to the general 

 free mode of life, however, occurs among the isopods, a few species 

 of which have adopted parasitism, and have become structurally 

 adapted to a parasitic life in a degree equal to that of some of the 

 entomostracans. This fact shows how readily metamorphosis can 

 crop out independently in species that have adopted a new way of 

 living. 



The isopods in general are a conservative group in which the young 

 hatch at a late stage of development with complete body segmentation 

 and most of the appendages present. Among those that have become 

 parasitic, however, varying degrees of adaptive metamorphosis occur 



