26 



ZOOLOGY 



SKC 



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

 lang). 



In the composite Ascidians, as mentioned 



1st 



periph 



stom. 



Fig. 72'). — Diagram of a zooid of a colony of Composite 

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

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

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

 zooids: end. endostyle ; <jld. digestive gland; (tn. nerve- 

 ganglion ; /(/. heart ; hyp. neural gland ; Inmj. languets ; 

 iitaiit. mantle ; or. ap. oral apertui'e ; oc. ovary ; periph. peri- 

 pharyngeal band ; ph. pliarj-nx ; rert. rectum ; .■<tom. stomach ; 

 te. testis; tent, tentacles; t.'st. test, or common gelatinous 

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



essential structure the zooids of such 

 resemble the simple Ascidians. 



in the summary, the 

 zooids are embedded 

 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 {Fsam- 

 mapilidium) the 

 gelatinous substance 

 is hardened by the 

 inclusion in it of 

 numerous sand- 

 gjrains. 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) 



