GONADS 



409 



end is a ciliated depression usually known as the olfactory 

 pit (plf. p). There is no auditory organ. The dorsal and 

 ventral nerves (Fig. 100 A, sn) remain separate, and do 

 not unite to form a nerve-trunk (compare p. 163). 



The sexes are distinct, and the metamerically arranged 

 gonads (Fig. 98-100, gon. g) are situated in pouches, the 

 cavities of which represent part of the coelome and which 

 project into the atrium on either side of the pharynx. When 



eL.a.0 



ph 



CLf.br.cL 



q/:br.a- -"O^ afhr.a. 



ep.port.v 



Fig. ioi. — Diagram of the vascular system o^ AviJ/hioxus. 

 of. hr, a, afferent branchial arteries ; hr. cl. gill-slits ; cp. interstinal capillaries ; 

 d. ao. paired dorsal aortee ; d. clo'. median dorsal aorta ; ef. br. a. 'efferent 

 branchial arteries ; hep. port, v. hepatic portal vein ; hep. v. hepatic vein ; int. 

 intestine ; Ir. liver ; ph. pharynx : s. int. v. sub-intestinal vein ; v. ao. ventra 

 aorta. (From Parker and Haswell's Zoology^ 



rip^j^ the eggs and sperms are shed into the atrium and find 

 their way out through the atriopore. 



The oosperm undergoes segmentation, and gives rise to a 

 simple larva, which gradually develops into the adult form. 

 The embryology of Amphioxus, which is very instructive, 

 will be referred to in greater detail in ChapterXI. 



