FOURTH REPORT OF THE DIRECTOR I907 1 59 



illustrations of a worm from the Hamilton group described by Hall 

 as Spirorbis angulatus from closely attached examples 

 showing but one or two entirely horizontal volutions. These silica 

 etchings show how^ quickly in later grow^th the tube departs from 

 the horizontal position and draws out into a loose spiral even when 

 not confronted by the necessity of keeping its feeding end on a level 

 with that of some companion organism [pi. 2, fig. 8-1 1]. 



The third of these combinations is illustrated b}^ a specimen for 

 which I am again indebted to Professor Calvin. A little colony of 

 Favosites has had its tentacular surface entirely overgrown with a 

 Stromatopora. Within the substance of . the Stromatopora is a 

 multitude of spiral worm tubes not stretched out into loose a'oIu- 

 tions as in the other instances mentioned, but keeping their two or 

 three volutions in close contact and resembling an Autodetus with- 

 out its external smoothly sloping surface. The edges of these 

 tubes are apparently always angular. These little worms have 

 started growth an}nA'here on the substance of the Stromatopora 

 and instead of growing like a Spirorbis with whorls broadly 

 attached for a turn or two, have coiled closely upward and ceased 

 growth in every case very abruptly. This case is singularly in- 

 structive as showing that the worm failed to keep pace in growth 

 with the coral and confessed its natural limitations of growth, while 

 in the other cases cited the worm apparently has had the ability to 

 adapt itself to this upward growth by stretching out its tube into 

 loose curves and keeping its aperture always clear at the surface. 

 The little Streptindytes compact us however was not equal 

 to this struggle for existence except as it planted its successors in- 



Stromatopora with embedded spiral annelid tubes. Streptindytes compactus. 

 located at various stages of the growth of the coral. The character of the annelid tube is 

 shown in the enlargement at the right (x s). The Stromatopora has entirely overgrown a 

 small Favosite* colony. Middle I'evonic. Iowa 



