46 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



post-abdomen. On the left, the two branches are united. In the 

 aggregated forms of other species of Cyclosalpa the posterior branches 

 of muscle IV (or its homolog in C. floridana) , at their point of union on 

 the mid line, give rise to a band of muscle which runs on the mid line 

 into the more or less developed post-abdomen. In the aggregated C. 

 virgula the corresponding muscle arises well on the right side, but has 

 a similar course into the base of the post-abdomen. Its position, on 

 the right side rather than in the median plane, is another indication of 

 the shifting of the whole muscular system toward the right. The 

 dorsal view (fig. 33, pi. 13) gives the best impression of this great 

 lateral distortion. Careful study of the origin and position of the 

 zooids upon the stolon might give the key to this distortion, but we 

 have not material to undertake this. 



The atrial muscles show a well-developed retractor on each side, 

 whose position and connection with the atrial sphincters, when care- 

 fully compared, is seen to have some resemblance to what is found in 

 the solitary form. The atrial retractor is not connected with the 

 basal two sphincters on either side. These basal atrial sphincters 

 are continuous on the right side, from the dorsal to the ventral 

 surface. On the left side they are interrupted, and in front of the 

 point of interruption the ends of the two are united into a common 

 strand, which runs toward but not into contact with body muscle 

 IV, suggesting comparison with the atrial retractors in the ag- 

 gregated forms of other species of Cyclosalpa. Comparison with 

 the solitary O. virgula indicates that that which corresponds to the 

 continuous atrial retractor in the solitary form is, in the aggregated 

 form, divided into two portions, proximal and distal, of which the 

 distal portion, connected with the delicate sphincters, is present on 

 both sides of the body, while the proximal portion, connected with 

 the heavier basal two sphincters, is seen only on the left side, and 

 even on this side is imperfectly developed. 



The oral muscles are essentially alike on the two sides, but are less 

 compact, and so more readily observed, on the right side. There is a 

 single strong oral retractor which, on the right side, passes backward 

 outside of and then joins the dorsal part of the intermediate muscle 

 and runs with it to its insertion upon body muscle I. On the left the 

 oral retractor passes over the intermediate muscle, but is not united to 

 either this or body muscle I. Four sphincters of the lower lip and one 

 sphincter of the upper lip are connected directly with the oral retrac- 

 tor on each side, as shown in the figures. The second sphincter of the 

 upper lip is independent of all other muscles. This is seen more 

 readily on the right side, where the oral muscles are not so crowded. 

 The third (last) sphincter of the upper lip is a branch of the inter- 



