vol. 2, ft. 2.] A TAXONOMIC STUDY OP THE SALPIDAE METCALF. 163 



atrium of Amphioxus and the "atrial" furrows of Balanoglossus. The 

 Larvacea show in their structure no indication of having had in their 

 ancestry Ascidianlike forms. All their structure may well have 

 been evolved during pelagic life. This is especially true of their 

 remarkable test ("house") which is their most specialized feature. 



The adoption of sedentary life led to change of form, accompanied 

 by loss of the tail with its axial skeleton, its elongated nerve tube 

 and its skeletal muscles, and to special development of the food 

 collecting organ, the pharynx with its gill basket. Increased size also 

 followed, and concomitantly the remnant of the nerve tube enlarged 

 into the cerebral ganglion. As is so usual with sedentary forms, 

 asexual reproduction was emphasized. Clavelina gives us our best 

 picture of this stage of evolution in the phylum. 



From some such early sedentary Ascidians apparently diverged 

 several lines of descent. By one or more of these the nonbudding 

 simple Ascidians arose, or they may have arisen before the budding 

 habit was developed. By another line Octacnemus arose. Still 

 another line, or possibly several independent lines, gave us the com- 

 pound Ascidians, from which in time arose the secondarily pelagic 

 Pyrosoma. The origin of Pyrosoma from compound Ascidians seems 

 indicated by the remarkable Distomid, Cyathocosmus mirabilis. 

 (See Oka, 1912 and 1913, also Metcalf and Hopkins, 1918). A final 

 line, of doubtful origin, which early branched dichotomously, de- 

 veloped, on the one hand, into the Doliolidae, on the other, into the 

 Salpidae. 



The presence of a sedentary stage in the ancestry of Doliolum and 

 Salpa is indicated by the possession by these two forms of features 

 first acquired as an adaptation to sedentary life, that is, the large 

 size, the short compact nervous system, the extensively developed 

 branchial basket, and the great development of asexual reproduction. 



Doliolum and Anchinia, returning to pelagic life, take with them 

 their large size, their compact ganglion, their complex pharynx and 

 highly developed atrium, all acquired under sedentary conditions. 

 They adopt the barrel-shaped form of body, moving the mouth and 

 atrial aperture to opposite ends of the body, modifying the sphincter 

 muscles of their two siphons into a series of hoop-like muscles, whose 

 contractions narrow the whole body, causing expulsion of water 

 from the atrial opening and this propelling the body forward. The 

 branchial basket moves backward from the two sides of the pharynx 

 to form a transverse lattice-work between the pharynx in front and 

 the atrium behind, which now has lost its peribranchial outgrowths. 



The structural modification of the Doliolidae from the Ascidian 

 condition is considerable, but their chief feature of specialization 

 is the great development and the unique character of their asexual 

 reproduction with migrating polymorphic buds. 



