XIII 



PHYLUM CHORDATA 



23 



periph 



CTlfi 



slom 



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 Avail alone is 



usually perforated 



by stigmata (stig.). 



An endostyle (end.) 



is present, and a 



peri -pharyngeal 



band : but there is P h 



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. 

 Sctlpa (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. 



te. 



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 ; Id. heart ; Inip. hypophysis ; lang. languets ; 

 ;,tant. mantle ; or. ap. oral aperture ; or. ovary ; periph. peri- 

 pharyngeal baud ; ph. pharynx ; net. rectum ; stom. stomach ; 

 t>-. testis; tent, tentacles; tst. test, or common gelatinous 

 mass ; T. d. vas' deferens. (After Herdman.) 



