XIII 



PHYLUM CHORDATA 



23 



,*, 



CTlft 



slant 



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 pos- 



terior wall alone is 



usually perforated -perish ^ p ' ^ cl 



by stigmata (stig.). 



An endostyle (end.) 



is present, and a 



Ci-pharyngeal 

 d; but there is 

 no dorsal lamina. 

 Doliolum moves 

 through the water 

 by the contractions 

 of the muscular 

 bands, which have 

 the effect of driving 

 the water back- 

 wards out of the 

 branchial sac. 



Salpa (Figs. 684- 

 685) is nearly allied 

 to Doliolum in its 

 external features 

 and internal struc- 

 ture. It has a fusi- 

 form body, usually 

 somewhat com- 

 pressed 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 separ- 

 ated from one another only by an obliquely running vascular 

 band, which represents the dorsal lamina of the fixed Ascidians, 

 and is frequently termed the branchia. 



til 



FIG. 682. Diagram of a zooid of a colony of Composite 

 Ascidians, in which the zooids are in pairs, as seen in a 

 vertical section of the colony, an. anus ; at. atrium ; at'. 

 atrium of adjoining zooid ; cl. cloaca common to the two 

 zooids ; end. endostyle ; gld. digestive gland ; gn. nerve- 

 ganglion ; ht. heart ; hyp. hypophysis ; lang. languets ; 

 mant. mantle ; or. ap. oral aperture ; ov. ovary ; periph. peri- 

 pharyngeal band ; ph. pharynx ; rect. rectum ; stom. stomach ; 



te. testis ; tent, tentacl 

 mass ; x. d. vas deferens. 



tst. test, or common gelatinous 

 (After Herdman.) 



