XIII 



PHYLUM CH(JRDATA 



26 



ap 



tt^S 



n^ 



o-to/n, int At 



noto 



Fig. 723.— Diagram of Appendlcularia from the right 

 side. an. anus; ht. licart ; mt. iutestiiie ; ac. iiorve ; 

 /!«'. caudal portion of nerve ; nc. f/a.' principal nerve- 

 ganglion ; ne. fjn.", ne. 'jii.'" first two ganglia of nerve of 

 tail ; noto. uotochord ; ««. oesophagus ; or. ap. oral aper- 

 ture ; oto. otocyst (statocyst) ; pen. M. peripharyngeal 

 baud \ph. pharynx ; tes. testis ; stig. one of the stigmata ; 

 stom. stomach. (After Herdman.) 



Among the simple Ascidians there is a considerable degree of 

 uniformity of structure, and little need be added here to the 

 account given of the 



example. The shape zyeribd ^^^..' 



vanes a good deal : it "" ■ ' ' - 



is sometimes cylindri- 

 cal, sometimes globu- 

 lar, sometimes com- 

 pressed; usually sessile 

 and attached by a 

 broad base, often with 

 root-like processes, but 

 in other cases {e.g. 

 Boltenia) elevated on 

 a longer or shorter 

 stalk. Most are soli- 

 tary: but some (the so- 

 called social Ascidians) 

 multiply by budding, 

 stolons being given off 

 on which new zooids 

 are developed, so that 



associations or colonies are formed ; but the connection between 

 the zooids is not close, and their tests remain distinct and 

 separate. The test varies considerably in consistency, being 



sometimes almost gelatinous, 

 transparent or translucent, some- 

 times tough and leathery, occa- 

 sionally hardened by encrusting 

 sand-grains or fragments of shells, 

 or by spicules of carbonate of 

 lime. Calcareous spicules may be 

 developed in the substance of the 

 mantle. The apertures always 

 have the same position and re- 

 lations, varying only in their 

 relative prominence. The pharynx 

 varies in its size as compared with 

 the rest of the internal parts, in 

 the position which it occupies 

 wdth regard to the various parts 

 of the alimentarv canal, and in 

 the number and arrangement 

 of the stigmata. The tentacles 

 are sometimes simple, some- 

 FiG. 724.— BotryUus vioiaceus. or. times compound ; and the 



oral apertures ; d. opening of common i i i • , 



cioacai chamber. (After Milne-Edwards.) dorsai lamina may or may not 



