TETHYODEA. 



93 



pharynx becomes perforated by an ever-increasing number of slits, 

 and thus gives rise to the branchial basket-work. 



The special form of the branchial basket-work presents numerous 

 modifications of systematic value. Not only is the external surface of 

 the branchial sac attached to the body-wall by trabeculse and bands, 



6 



FIG. 562. Appendicularia (Fritillarict) furcata. a, From the ventral side with the tail bent 

 forwards. GD, Genital glands ; M, muscles of the tail, b, From the ventral side after 

 the caudal appendage has been removed. 0, Mouth; End, endostyle ; Sp, the two 

 ciliated passages of the pharyngeal cavity; DW, the dorsal row of cilia; Oe, oesophagus; 

 Md, stomach ; Af t anus ; Dr, glands ; C, heart ; Ov, ovary ; T, testis. 



through which the blood passes, but the internal surface also often 

 presents folds and projections of varying form. Similarly the 

 branchial openings with which the wall of the pharynx is pierced 

 differ in size, number, and form ; they may be rounded, elliptical, or 

 even spirally coiled. 



The ciliated arrangements in the perforated branchial sac of the 



