86 UNITED STATES NATIONAL IVIUSEUM BULLETIN 251 



Figure 28. — Trididemnum cyclops Michaelsen: a, schema showing section of colony from 

 Gilbert Islands, sta. GOC-41; b, spicules from same colony, X 940; c, right side of thorax 

 of zooid of same colony (tr. m.= transverse muscle); d, abdomen of same zooid. 



tive zooids, where spicules are found much less dense than in other 

 portions. The superficial spiculeless layer is very prominent over 

 the whole surface of colonies; it is mainly milky white, but faintly 

 purplish brown in some parts along the periphery in some colonies, 

 and translucent, although a small amount of fine whitish sand 

 gi-ains are embedded in some places within the layer in the specimens 

 from G0a41. 



Spicules e32-53/i in diameter, about 40yLi on an average, distributed 

 evenly throughout test from bottom to surface, except the superficial 

 spiculeless layer, densely in specimens from B-8 or somewhat sparsely 

 in specimens from GOC-41, where the reddish-brown zooids are seen 

 through the test. Rays rather short, stout and bluntly pointed at 

 tip; 11-14 rays (specimens from B-8) or 12-17, about 14 on an aver- 

 age (specimens from GOCMrl), on the equatorial plane. Many 

 zoochlorellae, 14-1 ()/x in diameter, found in thoracic lacunae. 



Zooids rather small, contracted thorax about 500/1, and abdomen 

 about 450/1 in length. Retractile muscle short in specimens from 

 GOC-41, but very long, approximately twice as long as contracted 



