vol. 2, pt. 2.] A TAXONOMIC STUDY OF THE SALPIDAE METCALF. 107 



The atrial siphon and its musculature (fig. 100) in. the aggregated 

 lasis zonaria resembles, but is not identical with, that of the solitary 

 form. Dorsally are similar triangular thickenings of the test. One 

 sees a strong atrial retractor (a. r.), four ventral atrial sphincters 



0.1: 



Fig. 99.— Iasis zonakia, aggregated form, the left half of the oral siphon, seen from the 

 inner side. it is shown slightly strf.tcheb open. x 18 diameters. (drawn by hoyt s. hop- 

 KINS.) 



{v. s. 1-4), the last three connected with the retractor and the first 

 continuous with the admarginal dorsal sphincter. The second dorsal 

 sphincter is not connected with the second ventral sphincter, differing 

 in this regard from the corresponding muscle in the solitary form. 



a>.r. 



Fig. 100.— Iasis zonauia, aggregated form, atiual muscles. 



Like the third dorsal sphincter in the solitary form, the second dorsal 

 sphincter breaks up distally into a number of fibres, forming a com- 

 plex whose pattern differs in different individuals, and may even be 

 different on the two sides of the same siphon. The atrial musculature 

 in both solitary and aggregated forms of Iasis is very different from 



