2(5 



ZOOLOGY 



be divided up into a sy.stem of lobes or langn.ds (Fig. 725, 

 king). 



In the composite Ascidians, as mentioned 



tst 



stom 



Fjg. V25. — Diu^am of a zooid uf a colony of Composite 

 i^scidianS; in which the zooids are in pairs, as seen in a 

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

 atrium of adjoining zooid; c/. cloaca common to tlic fwo 

 zooids ; end. endostyle ; j/Zi/. digestive gland; <)n. nerve- 

 ganglion ; }i1. heart ; hyii. neural gland ; laaf;. langucts ; 

 'iiiant. mantle ; oi\ fip. oral aperture ; or. ovary ; jierip/i. peri- 

 pharyngeal band ; j>li. pharynx ; ;cc/. rectum ; ^tom. stomach ; 

 fe. testis ; tent, tentacles; if:/, test, or common gelatin(jus 

 mass; i'. rf. vas deferens. (After Herdnian.) ■ 



essential structure the zooids of such 

 resemble the simple A.scidians. 



in the summary, the 

 zooids are embedded 

 in a common gelatin- 

 ous mass formed of 

 their united test.s. 

 The gelatinous 

 colony thus formed is 

 sometimes fla,t and 

 encrusting, some- 

 time.s branched or 

 lobed, sometimes ele- 

 vated on a longer or 

 shorter stalk. In cer- 

 tain forms {Psam- 

 mapilidiiom) the 

 gelatinous substance 

 is hardened by the 

 inclusion in it of 

 n u m e r o u s s a n d - 

 grains. The ar- 

 rangement of the 

 zooids presents great 

 differences. Some- 

 times they occur 

 irregularly, dotted 

 over the entire sur- 

 face without exhi- 

 biting any definite 

 arrangement ; some- 

 times they are ar- 

 rows or 

 in 



DoiryUus" (Fig. 724) 

 they form star- 

 shaped, radiating 

 sets around a com- 

 mon cloacal cham- 

 ber into which the 

 atrial apertures of 

 the zooids lead, 

 while the oral aper- 

 tures are towards 

 their outer ends. In 

 colonies (Fig. 725) 



ranged m 



