104 



BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



Fig. 94. — Iasis zonaria, solitary form, sagittal sec- 

 tion OF THE ATRIAL APERTURE AND THE ADJACENT TIS- 

 SUES, x 8 diameters. (Drawn by Hoyt S. Hopkins.) 



opening, this second ventral sphincter is continuous with the third 



dorsal atrial sphincter (a. s. 3) which starts as a single band, but soon 



breaks up into fine strands 

 that form something of a 

 reticulum. Several strands 

 from this reticulum run for- 

 ward to join the atrial re- 

 tractor. These are independ- 

 ent of the main trunk of the 

 third dorsal sphincter, and 

 may represent a fourth dorsal 

 atrial sphincter. The third 

 ventral atrial sphincter (v. s. 3) 

 has no connection with mus- 

 cles of the dorsal atrial lip, but 

 is united by two branches to 



the middle portion of the atrial retractor. The dorsal lip of the atrial 



aperture is in two parts. The first, a slightly developed flap, at a 



lower level, bears dorsal sphincter No. 1 . 



Dorsal sphincter No. 2 is not connected 



with any other muscle. It lies in the 



upper division of the upper lip. It is 



interrupted on the mid line, its two ends 



being bent forward . The upper portion 



of the dorsal lip has its test developed 



to form two triangular stiffened areas, 



just beneath which lies the reticulum 



formed by the strands of the third and 



fourth ( ?) dorsal atrial sphincters. The 



vertical section of this region shown 



in figure 94 should help to make these 



structures clear. 



The gut forms a rather close circular 



loop (figs. 90 and 94), but is much less 



compact than in Apsteinia and Salpa 



proper. It is much like Apstein's figure 



of the gut in Brooks i a rostrata (fig. 23, 



p. 51). 



In the character of its eye and the 



outgrowths from its ganglion (figs. 95 



and 96), the soli! ary form of Iasiszonaria 



is one of the most aberrantof thesolitary 



Salpidae. Indeed, with the exception of 



RiUeria hexagona, it is the only species in the solitary form of which 



these structures depart markedly from the usual type. The eye has 



Fig. 95.— Iasis zonabia, .solitary form, 

 dorsal view of the eye, the gan- 

 glion, the nerves, the lateral out- 

 gbowths from the ganglion, and the 

 posterior part of the ciliated fun- 

 nel. x 57 diameters. from metcalf 



