xm 



PHYLUM CHORDATA 



25 



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 



eri h 



07* 



FIG. 733. Botryllus violaceus. 



or. oral apertures ; d. opening of 

 common cloacal chamber. (After 

 Milne-Edwards.) 



or regular groups ; in 

 Botryllus (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 



slon 1 



/it - 



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 ; c/. cloaca 

 common to the two zooids ; end, endostyle ; gld. 

 digestive gland ; gn. nerve-ganglion ; ht. heart ; 

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

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

 band ; ph. pharynx ; red. rectum ; stom. stomach ; 

 te. testis ; tent, tentacles ; tst. test, or common 

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

 man.) 



colonies (Fig. 734) resemble 

 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 



