584 



A. WILLEY. 



inwards as a ridge on each side, delimiting a dorsal pars 

 nutritoria from a ventral pars respiratoria. The gill- 

 openings are simple clefts destitute of tongue-bars, but the 

 gill-arches which bound them are considerably folded, and 

 exhibit a bilaterally symmetrical structure being incompletely 

 divided by a deep median groove into right and left halves. 

 The gill-arches are somital, the gill-slits intersomital, in their 

 topographical relation to the myotomes. 



The primitive condition of the pharynx of Amphioxides 



Fig. 1. — A. pelagic us. Section through the middle of the 

 branchial region. After Goldschmidt. ao. Aorta, h.a. Branchial 

 artery. Jjm. Branchial muscle, 'p.n. Pars nutritoria of pharynx. 

 <p.r. Pars respiratoria. 



would therefore consist in the median ventral series of un- 

 paired gill-slits, the partial separation of the ventral pars 

 respiratoria from the dorsal pars nutritoria, and, con- 

 sequent thereupon, the perforation of a sinistral mouth into 

 the dorsal division, and the development of a ciliated 

 glandular organ, the endostylc, opposite to the mouth, also 

 leading up to the dorsal division. The club-shaped gland is 



