106 



BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



posterior branch, the latter, 

 of the posterior branch of l! 



em. 



Fig. 97.— Iasis zonaeia, aggregated zooid, 

 dorsal view. x 3 diameters. (drawn 

 bt Hoyt S. Hopkins.) 



sphincter complex is reduced; 

 the. dorsal portion reduced. 



Tho condition of the oral m 

 viduals anions; mv material 



though short, probably the homolog 

 io last body muscle in the aggregated 

 zooids of other species. 



The intermediate muscle is well de- 

 veloped ventrally, extending as near to 

 the mid-ventral line as do the body 

 muscles, but dorsally it passes but a 

 little way beyond the angles of the 

 mouth. Streiff interprets the inter- 

 mediate muscle as double, an interpre- 

 tation which seems to be borne out by 

 comparison with some species still to 

 be described. 



The oral musculature (fig. 99) shows 

 a short oral retractor, which gives rise, 

 at the angle of the mouth, to a wide 

 admarginal sphincter muscle in each 

 strongly incurved lip. Across the angle 

 of the oral siphon lies a band of dark- 

 colored muscle, comparable to that 

 seen in a similar position at the distal 

 end of the oral siphon in the solitary 

 form of this species. Comparisons, 

 especially with the C} 7 closalpas, will 

 show how various are the interrela- 

 tions of the intermediate muscle and 

 the more basal sphincters of the lips, 

 especially the upper lip. In several 

 species the ventral portion of this 

 Iasis hexagona, on the other hand, has 



usculature, in the very numerous indi- 

 , is considorablv different from that 



. aggregated zooid, seen from the right side. x 

 by Hoyt s. Hopkins.) 



figured and described by Streiff. I find no delicate second sphincter 

 in the lower lip, such as he describes, and the dark-colored muscle at 

 the angle of the mouth is situated more distally in my specimens. 



