XIII 



PHYLUM CHORDATA 



21 



FIG. 679. Appendicularia (Oikopleura) in 



(From Herdman, after Fol.) 



House." 



of the mouth. This leads into a tolerably wide pharynx (Fig. 

 680, ph.), in the ventral wall of which is an endostyle similar to 

 that of the simple 

 Ascidian, but com- 

 paratively short. 

 Round the pharynx 

 there run two bands 

 covered with strong 

 cilia fheperipha ryn- 

 geal lands. On the 

 ventral side of the 

 pharynx there are 

 two ciliated openings 

 the stigmata (stig.) 

 which communi- 

 cate with the exterior by short passages the atrial canals, 

 situated on either side behind the anus. The axis of the tail is 

 occupied by a cylindrical rod the notochord (noto.). 



A remarkable peculiarity of Appendicularia is the power which 

 it possesses of secreting from the surface a transparent envelope 

 (Fig. 679) in the interior of which the animal can move freely. 

 This structure the house as it is called is soon thrown off, and 

 a new one developed in its stead. It represents the test or 



tunic of the simple 

 Ascidian, though it 

 does not appear to 

 contain cellulose. 



Among the simple 

 Ascidians there is a 

 considerable degree 

 of uniformity of struc- 

 ture, and there is not 

 much that need be 

 added here to the ac- 

 count given of the 

 example. The shape 

 varies 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 solitary ; but some multiply 

 by budding, stolons being given off on which new zooids are 

 developed. The test varies considerably in consistency, being some- 



noto 



FIG. 680. Diagram of Appendicularia from the right- 

 hand side. an. anus ; ht. heart ; int. intestine ; ne. nerve ; 

 ne.' caudal portion of nerve ; ne. gn. principal nerve- 

 ganglion ; ne.gn.'ne. gn." first two ganglia of nerve of 

 tail ; noto. notochord ; as. oesophagus ; or. ap. oral aper- 

 ture ; oto. otocyst ; peri. bd. peripharyngeal band ; ph. 

 pharynx ; tes. testis ; stig. one of the stigmata ; stom. 

 stomach. (After Herdman.) 



