vol. 2, i>t. 2.] A TAXONOMIC STUDY OF THE SALPIDAE METCALF. 157 



siphon becomes two-lipped and in all these the atrial retractor is 

 well developed. 



The oral siphon in all forms is decidedly two-lipped and the oral 

 musculature does not in any species show a simple series of regular 

 circular sphincters. 



Apparently the oral muscles, intermediate muscles, body muscles, 

 and atrial muscles, should be regarded as serially homologous with 

 one another, as Doliolum suggests. The intermediate muscle is de- 

 veloped in some species, in general the more primitive, to function 

 with the body muscles. In the more modified species, especially 

 those in which the musculature is reduced, the intermediate muscle 

 functions with the oral muscles. Similarly between the body 

 muscles and the definitive atrial sphincter muscles are one or often 

 two bands which are intermediate in character between body muscles 

 and atrial sphincters. A sharp fine of demarcation between the two 

 series of muscles does not generally exist. 



There is a reduction in the eyes of the aggregated zooids in one of 

 the more highly modified of the Salpidae, the anterior portion of the 

 large dorsal eye seen in all other species, being wholly wanting in 

 Thalia. 



In Thetys the basal and terminal portions of the large dorsal eye 

 are less distinct from each other than in most species, the eye being 

 very compact, but both portions are recognizable in the arrangement 

 of the rod and pigment cells. 



The histological condition of the rod-cells of the large dorsal eye 

 of the aggregated zooids shows degeneration in the more modified 

 sub-genera, as Thetys and Pegea. Their condition in Traustedtia is 

 not known. The eyes within the ganglia of some species show 

 similar histological degeneration, as Pegea, Thetys, Ritteria hexagona, 

 and even Salpa fusiformis and S. maxima. 



The neural glands, also, depart from the usual condition, in the 

 more modified species, being united into one below the ganglion in 

 Iasis and Thetys, and being wholly absent in Thalia. Even in Salpa 

 cylindrica, not a highly modified species in general, the disks of the 

 gland are absent and the ducts are only slightly developed. The 

 outgrowths from the ganglion in connection with the neural glands 

 are reduced to a single pair or are even unrecognizable in some of 

 the more modified subgenera. 



The conclusions as to relationships among the Salpidae, which I 

 have reached from a study chiefly of the muscles, gut, and eyes, are 

 shown graphically, though too definitely, in the accompanying chart. 

 Upon the exact points of origin of the divergent lines of decent I 

 would by no means insist, but the vividness of the graphic method 

 of presentation outweighs the overemphasis upon some of the details 

 which is necessarily involved in such a chart. 



