PHYLUM CHORDATA 



27 



In the free-swimming pelagic DoUolum (Fig, 726) the shape is 

 widely difterent from that of the ordinary fixed forms. The body 

 is cask-shaped, surrounded, as by hoops, by a series of annular 

 bands of muscular fibres (vius. bds.). The oral and atrial apertures 

 {or. ap., atr. ap.) instead of being situated near together at the same 

 end of the body, are placed at opposite extremities, and the 

 relations of the various organs have undergone a corresponding 

 modification. The test is thin and transparent. Surrounding 

 each opening is a series of lobes — the oral and atrial lohcs — in 

 which there arc sense-organs ; and the first and last of the 

 muscular hoops serve as sphincters for the two orifices. The oral 

 aperture leads into a wide pharyngeal sac (ph.) occupying at least 

 the anterior half of the body ; its posterior wall alone is usually 

 perforated by stigmata {diij.). An endostyle [end.) is present, and 



7/iu.s.bds 



trap 



ph *^4? ^"^ test 



storrv 



Fn;. 72rp. — Doliolum. Diaj,nMm of the .sexual furrn. air. c.jt. ;itrial aiiertiire sinntuinied liy 

 lubes ; air. car. atrial cavity ; >l. ihc. dursal tubercle ; e>id. cndu.4tylc ; hi. heart ; iai. intestine ; 

 iiiuii. b</s. muscular bands ; m;. gn. nerve-ganglion ; or. ap. oral aperture ; or ovary ; peri. bd. 

 peripharyngeal band ; 'ph. pharynx ; silg. stigma ; iiiom. stomach ; test, testis. (After Herd- 

 man.) 



a peripharyngeal band ; but there is no dorsal lamina. Doliolum 

 moves through the w^ater by the contractions of the muscular 

 bands, which have the effect of driving the water backwards out 

 of the branchial sac. 



Salpa (Figs. 727-728) is nearly allied to Doliolum in its 

 external features and internal structure. It has a fusiform body, 

 usually somewhat compressed laterally, and with the oral and 

 atrial cavities nearly terminal ; but the muscular bands do not 

 form complete hoops. The pharyngeal and atrial cavities take up 

 the greater part of the space in the interior of the body, where 

 they form an almost continuous cavity, being separated from one 

 another only by an obli<pieIy running vascular band, which repre- 

 sents the dorsal lamina of the fixed Ascidians and is frequently 

 termed the hninchia. 



Octacnermcs, sometimes regarded as allied to Salpa, appears to be 



