146 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



At the base of the atrial siphon is another muscle nearly as well 

 developed as body muscle IV and seeming to belong to the body- 

 muscle series, but its position at the base of the siphon, and the fact 

 that body muscle IV in its branching resembles the last body muscle 

 of the aggregated zooids of numerous species, suggest that the muscle 

 in question may better be interpreted as a basal atrial -muscle. 

 Compare the conditions in the Cyclosalpas, both solitary and aggre- 

 gated forms. Without insisting upon its homologies I label it in the 

 figures as body muscle V. 1 This muscle is branched, a small band 

 arising from its upper portion, on each side, and running back to 

 unite with the delicate sphincter ol the atrial siphon near its point 

 of union with the very delicate uppermost branch of the posterior 

 division of body muscle IV. 



Between body muscles IV and V there is a minute oblique muscle 

 on each side of the dorsal mid line not attached at either end to any 

 other muscle. 



There are two atrial sphincter muscles, an admarginal one and a very 

 delicate submarginal one, which, on each side, is fused for a very short 

 distance with the admarginal muscle. There is a short portion of a 

 third sphincter across the ventral surface of the siphon below, anterior 

 to, the delicate sphincter. The axial tube of the lateral "tentacle" (5) 

 of the atrial siphon arises from the mantle exactly at the level of the 

 admarginal atrial sphincter muscle (fig. 35), but there is no continu- 

 ation of the muscle into the " tentacle" as the figures seem to indicate. 



The oral musculature, like that in Pegea, is weakly developed. 

 The intermediate muscle is present as a short band running obliquely 

 internal to the two oral retractors. The two oral retractor muscles 

 are horizontal or nearly so. The more ventral divides into two short 

 branches which form abortive sphincter muscles, one at the base of 

 the upper lip and the other at the base of the lower lip. The dorsal 

 oral retractor muscle divides into three branches the upper of which 

 forms a similar abortive first sphincter of the upper lip, while the 

 more ventral two form the narrower and the wider complete sphinc- 

 ters of the incurved lower lip. There are no dorsal horizontal bands 

 between the oral and body muscles. 



The gut is in the form of a vertically placed elongated loop, much 

 more compact than in Cyclosdlpa affinis (pi. 3, fig. 10), less compact 

 than in Thalia (fig. 104, p. 111). 



The stolon is curved around the base of the intestinal loop, making 

 about two-thirds of a circle, the two ends, however, being at widely 

 different levels, thus forming a partial spiral. 



1 The distinction between postorior body muscles and basal atrial muscles is doubtless not a fundamental 

 one. The conditions, especially in Trauxtedtia and Pegea, seem to indicate that they are all serially hom- 

 ologous. The samo muscle which in one species is developed as a body muscle may, in another species, 

 serve as a basal atrial muscle. 



