XIII 



PHYLUM CHORDATA 



25 



tsl 



or lobed, sometimes elevated on a longer or shorter stalk. In 



certain forms the gelatinous substance is hardened by the inclusion 



in it of numerous sand-grains. The arrangement of the zooids 



presents great differences. 



Sometimes they occur 



irregularly, dotted over 



the entire surface without 



exhibiting any definite 



arrangement ; sometimes 



they are arranged in rows 



d r h - 



end 



slom. 



01' 



FIG. 733. Botryllus violaceus. 



or. oral apertures ; cl. opening; of 

 common cloacal chamber. (After 

 ]\lilne-Edwards.) 



or 



in 



regular groups ; 

 Botrytt-us (Fig. 733) they 

 form star-shaped, radiating 

 sets around a common 

 cloacal chamber into which 

 the atrial apertures of the 

 zooids lead, while the oral 

 apertures are towards their 

 outer ends. In essential 

 structure the zooids of such 

 colonies (Fig. 734) resemble 



FIG. 734. Diagram of a zooid of a colony of Com- 

 posite 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 ; h't. heart ; 

 hyp. neural gland ; lang. languets ; mant. mantle : 

 or. ap. oral aperture ; ov. ovary ; periph. peripharyngeal 

 band ; ph. pharynx ; reel, rectum ; stom. stomach ; 

 te. testis ; tent, tentacles ; 1st. test, or common 

 gelatinous mass ; v. d. vas deferens. (After Herd- 

 man.) 



the simple Ascidians. 



In the free-swimming pelagic Doliolum (Fig. 735) the shape is 

 widely different from that of the ordinary fixed forms. The body 



