XIII 



PHYLUM CHORDATA 



137 



.i.c 



the spinal nerves unite instead of remaining separate. The eyes 

 are vestigial and sunk beneath the skin, and the auditory organ 

 (Fig. 818) has only a single semi- 

 circular canal, which, having an 

 ampulla at each end, probably 

 represents both anterior and pos- 

 terior canals. 



Bdellostoma has a persistent 

 pronephros in the form of a 

 paired irregularly ovoidal body 

 situated just above the heart and 

 consisting of a large number of 

 tubules richly branched peripher- 

 ally : the nephrostomes open into 

 the pericardium. The tubules 



FIG. X18. Auditory organ of Myxine. 

 amp., amp', ampullae ; end. s. endolym- 

 phatic sac ; s. c. semicircular canal ; utr. 

 sac. utriculo-sacculus. (After Retzius.) 



FIG. 819. A, portion of kidney 



ureter ; 6, urinary tubule ; c, 

 Malpighian capsule ; d, afferent 

 artery ; f, efferent artery. 

 Compara- 



do not communicate with the 

 pronephric duct in Myxine, but end blindly : 

 in Bdellostoma they open into an incom- 

 plete longitudinal duct, which does not 

 communicate with the permanent kidney- 

 duct. The functional kidney is the mesone- 

 phros, and is specially interesting from the 

 fact that in Myxinoids it retains in the adult 

 its primitive segmental arrangement. The 

 ureter (pronephric duct, Fig. 819, a) sends 

 off in each segment a coiled tubule (6) with 

 a single Malpighian capsule (c), into which 

 a branch from the aorta (d) enters and 

 forms a glomerulus. 



Myxine is hermaphrodite, the anterior 

 part of the gonad being ovary, the posterior 

 testis : in some the ovary is mature and 

 the testis rudimentary, in others the oppo- 

 site condition holds good, so that, while 

 hermaphrodite, each individual is either 

 predominantly female or predominantly 

 male. The eggs of both genera are of 

 great proportional size, and those of Myxine 

 are enclosed when laid in a horny shell 

 bearing numerous hooked processes at each 

 pole; by means of these the eggs are en- 

 tangled together, and probably also attached 

 to seaweed. 



In Bdellostoma stouti, the only Myxinoid 

 of ^^ &* development is known, the 

 eggs are elongated and cylindrical, and 



^^ & ^ quantity Q f f ood . vo l k . 



The segmentation is meroblastic, being 



