60 



BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



throughout the dorsal half of their course, III and IV converging to 

 meet them on the dorsal surface, making a broad common trunk. 

 Similarly V and VI unite dorsally into one broad muscle. Muscle 

 VI has an anterior branch in front of the gut and a posterior branch 

 which joins its fellow of the other side at the base of the atrial siphon, 

 giving rise here, on the midline, to a median muscle, which runs into 

 the base of the postabdomen (fig. 33, B). This is clearly the homolog 

 of the visceral muscle we have emphasized in our description of the 

 aggregated Cyclosalpas. No muscles are described arising from the 

 lower part of the anterior branch of body muscle VI and passing into 

 the post abdomen, this agreeing with the aggregated Cyclosaljia 

 virgula. Just above its point of branching, body muscle VI connects 

 with the well-developed atrial retractor muscle. In an older zooid 



FlG. 33.— RlTTERIA AMBOINENSIS, YOUNG AGGREGATED ZOOID FROM THE STOLON: A, DORSAL VIEW; 

 B, VENTRAL VIEW. X 49 DIAMETERS. FROM lULE (1910). 



Ihle shows the atrial sphincter distinct at its base from body muscle 

 VI. Body muscle VI corresponds closely in its position, branches, 

 and connections with body muscle III of Cyclosalpa floridana and 

 body muscle IV of the other Cyclosalpas. In the solitary forms of 

 the Ritterias there is an increase in the number of body muscles. 

 The same is seen to be true of the aggregated zooids in R. arriboinensis 

 and it is also true of the aggregated form of R. hexagona (fig. 41, p. 67), 

 these being the only species of Ritteria whose aggregated zooids 

 are known. 



On the right side the arrangement of the body muscles is slightly 

 different (figs. 32 and 33 A), but their number is the same. There 

 is considerable asymmetry, both at their dorsal and theirventral ends, 

 between the body muscles of the right and left sides. At the base of 



