vol. 2, pt. 2.] A TAXONOMIC STUDY OF THE SALPIDAE METCALF. 13 



a little posterior to the place of branching of the dorsal oral retractor. 

 The ventral retractor is here internal to the dorsal retractor. At the 

 point whero the two retractors are at the same level, the ventral 

 retractor divides, giving rise to a ventral branch, which forms the 

 third sphincter of the lower lip (I. 3), and to a wider dorsal branch 

 which after a very short course divides to form the third (u. 3) and 

 fourth (u. 4) sphincters of the upper lip,. We may describe the oral 

 muscles as made up of two retractor systems, one consisting of the 

 dorsal retractor and the anterior two sphincters of each lip, the sec- 

 ond consisting of the ventral retractor, the third sphincter of the 

 lower lip and the third and fourth sphincters of the dorsal lip. Well 

 up on the side of the fourth sphincter of the dorsal lip there arises 

 a muscle band, the dorsal horizontal muscle, which runs backward 

 and dorsal-ward to unite with the dorsal end of the intermediate mus- 

 cle, the fused muscle continuing its course postero-dorsally to a point 

 at the side of the ganglion. 



The atrial muscles in the solitary form of this species consist only 

 of a series of sphincters which are complete hoops. The anterior 

 atrial sphincter is a strong band and may function with the body 

 muscles in expelling water from the body, but its position, well out 

 on the base of the atrial siphon, and its complete hooplike character, 

 make it natural to assign it to the atrial series. The adjacent sphinc- 

 ter is peculiar in the fact that its ventral portion bends forward, runs 

 across the basal sphincter on the inside and crosses the ventral line 

 well toward the last body muscle, at the level of the posterior ends of 

 the two intestinal caeca. A higher development of a somewhat simi- 

 lar arrangement in the solitary form of Cyclosalpa virgula (pi. 11, fig. 26) 

 gives the diagonal course of the ventral end of this muscle an atrial 

 retractor effect. In Cyclosalpa pinnata this effect is hardly secured. 

 The rest of the atrial sphincters are very delicate. They branch and 

 the branches from one sphincter unite with other sphincters, forming 

 a slight irregular network. 



The relative strength of the several muscles is correlated with the 

 amount of work they do. The strong body muscles, by vigorous 

 contraction, expel the water from the pharyngeo-atrial chamber 

 within the body. Those who have observed living Salps, describe 

 this contraction as ejecting water with considerable violence, throw- 

 ing'the whole Salpa forward a considerable distance. The oral sphinc- 

 ters have to resist the pressure of this water, preventing it from flow- 

 ing out through the mouth. They are so much weaker than the com- 

 bined strength of the whole series of body muscles that they would be 

 unable successfully to resist the pressure of water caused by the con- 

 traction of the body muscles, were it not that the oral sphincters are 



