32 ANIMAL AGGREGATIONS 



the nests of other fishes (Reighard, 1920). Many similar examples 

 could be given for other nests, such as those of ants and termites. 



5. Paroecia, or neighborly groups, are formed in which the less 

 conspicuous animal species finds protection from the other without 

 occupying a part of its nest. Thus, small fishes are frequently as- 

 sociated with medusae or with the Portuguese man-of-war Physalia; 

 while many animals, such as fish, worms, snails, and starfish, have 

 similar relationships with coral colonies. 



6. Metrokoinia occurs in ants when the fertilized female of one 

 species who has lost the ability to start a new colony joins herself 

 with the fertilized female of another species that has retained this 

 power, and is thus associated with a colony development which she 

 would be unable to secure alone, and to which she contributes Uttle 

 or nothing. This relation has been described for Strangylognathus 

 testaceus Sch., which has lost the power of colony formation, living 

 in mixed colonies with Tetramorium caespitum L. 



7. Irreciprocal sym porta occur when one animal species attaches 

 itself to the surface of another without becoming parasitic and with- 

 out contributing aid to the animal on whose back it grows. This 

 relationship may exist between barnacles growing on whales, be- 

 tween hydroids and crabs, and between stalked protozoans, such as 

 peritrichs and suctorians, and the snails, crustaceans, or hydroids 

 supporting them. 



8. Syncollesia are cemented societies in which one animal cements 

 into its own covering the case or shell of another species of animal 

 without killing off the original owner. Small mussels {Sphaeridae) 

 and snails may be worked into the cases of caddis-fly larvae. 



9. Parachorium is the name given to the relationship that exists 

 when one animal lives within the body of another without being 

 parasitic upon it. Hydroids, sea anemones, polychaete worms, ophi- 

 urids, and crustaceans live in the canal systems of sponges; and Pin- 

 notheres, a crab, lives in the mantle cavity of Mytihis, the sea mussel. 



10. Parasitism is not easily separated from several of the preced- 

 ing categories. A parasite, in the restricted sense used here, obtains 

 its nourishment, at least, from the host with whose continued exist- 

 ence the parasite is more or less closely bound. Frequently the nour- 



