26 



ZOOLOGY 



SECT. 



slom, 



be divided up into a system of lobes or languets (Fig. 725, 



lang.). 



In the composite Ascidians, as mentioned in the summary, the 



zooids are embedded 



tst in a common gelatin- 



ous mass formed of 

 their united tests. 

 The gelatinous 

 colony thus formed is 

 sometimes flat and 

 encrusting, some- 

 times branched or 

 lobed, sometimes ele- 

 vated on a longer or 

 shorter stalk. In cer- 

 tain forms (Psam- 

 mapilidium) the 

 gelatinous substance 

 is hardened by the 

 inclusion in it of 

 numerous sand- 

 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- 

 ranged in rows or 

 regular groups ; in 

 Botryllus (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) 



te. 



FIG. 725. 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 ; lit. heart ; hyp, neural gland ; lamj. languets ; 

 mant. mantle ; or. up. oral aperture ; ot\ ovary ; periph. peri- 

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

 te. testis ; tent, tentacles ; tut. test, or common gelatinous 

 mass ; v. d. vas deferens. (After Herdman.) ; 



essential structure the zooids of such 

 resemble the simple Ascidians. 



