PROTOZOA AND OTHER ANIMALS 919 



sea urchins, and flagellates in termites and roaches. The former is inter- 

 esting also from the standpoint of the relationship of inquilines and 

 free-living forms; the sea-urchin intestine is one of the least specialized 

 environments of its type. The flagellates, however, occupy one of the 

 most specialized of symbiotic habitats. Not only are the circumstances 

 under which they live and are transmitted exceptional, but the hind-gut 

 of the host has actually undergone structural modification to accom- 

 modate them. 



CILIATES OF SEA URCHINS 



Faunules of ciliates occur in the greater number of sea urchins that 

 have been examined, but there are some without any. Uyemura ( 1934), 

 giving positive reports from eight species of sea urchins of Japan, found 

 none in Brissus agassizi. At Amoy, Nie (1934) found none in Tem- 

 nopleurus toreumaticus. Of the species at Yaku Island, Japan, Yagiu 

 (1935) found two uninfected: Colobocentrotus mertensii and Cidaris 

 {Goniocidaris) biserialis. Powers (1935) found no faunule in Eucidaris 

 trihulotdes at Tortugas, nor were ciliates present in members of the 

 genus Afhacia at Beaufort, Woods Hole, and Naples (1933a), in spite 

 of association with infected species. Why a few species possess no 

 faunules, while so many have ciliates in abundance, is an interesting ques- 

 tion. 



The intestinal faunules of sea urchins consist mostly of ciliates, which, 

 in whatever part of the world the host occurs, are members of a number 

 of characteristic genera. Most of them are holotrichs; outside of this 

 group are a few species of Metopus and one of Strombilidium, hetero- 

 trichs with many free-living congeners. There are sometimes as many 

 as twelve distinct species in eight genera in Strongylocentrotus pur- 

 puratus (Lynch, 1929); and Yagiu (1933, 1934) found twelve species 

 in Anthocidaris crassispina. On the other hand, from S. jranciscanus in 

 Japan, Yagiu (1935) reported only Conchophthirus striatus. Four or 

 five species is perhaps the average infection. The occurrence of amoebae, 

 Chilomastix echinorum (Powers, 1935), nematodes, and rhabdocoeles 

 {Syndesmis) is much less prominent than that of ciliates. 



In given hosts there is variability in the occurrence of different species 

 of ciliates. Some occur in abundance almost or quite universally; others 

 have a lower incidence, some being of rare occurrence. Plagiopyla minuta 



