400 



CEPHALOCHOKDA 



[CH. 



the myocoelic cavities (d.f.c. Fig. 190). 1 There are also low fin 

 folds projecting from the sides of the atrial cavity and constituting 

 the lateral or metapleural fins (Fig. 189), and a median ventral 

 fold between the atrial pore and anus, called the ventral fin 

 (Fig. 185). The dorsal and ventral fins are stiffened by a number 

 of gelatinous rods, and those in the dorsal fin are developed 

 from thickenings of the cavities described above. 



The alimentary canal of Amphioxus is a perfectly straight tube 

 consisting of stomodaeum or mouth gut, pharynx or 

 branchial gut, and intestine or digestive gut. The pharynx 



no 



FIG. 196. Portion of transverse section through the pharynx of Amphioxus, to 

 show position of excretory tubule. After Weiss. 



ao. Left aorta. at. Atrial cavity. at.e. Atrial epithelium. c. Coelom. 

 ch. Notochord. i.m. Intercoelic membrane, l.d. Dorsal wall of atrial 

 cavity. npk. Nephridium. p.b. Gill-bar. ph.e. Epithelium 



of hyperpharyngeal groove. ph.f. Fold attached to gill-bar containing 

 branchial coelomic canal, s.cft. Sheath of notochord. t.b. Tongue-bar. 



has along both dorsal and ventral middle lines grooves lined with 

 cilia connected with each:, other by a circular groove just inside 

 the velum or true mouth. The ventral groove is called the 

 endostyle or hypopharyngeal groove, the upper groove is 

 termed the hyperpharyngeal, and the connecting groove the 

 peripharyngeal band (Figs. 188, 189, 191). The function of 

 these grooves is curious. The endostyle produces a cord of mucus 

 which is worked forwards by its cilia and pressed up the sides of 

 1 This supposition has recently been proved to be correct at least in the 

 case of one species, 





