47 



are, as a rule, only six pairs, the elongated external ducts pass 

 backwards, and open by a common pore on each side. The genus 

 ParamyxwM (Fig. 31) shows an admirably intermediate stage in 

 the evolution of this secondary arrangement (Dean [110]). 



A. 



FICJ. 29. 



MI/,I hie glutinosa, L. A, left-side view of the anterior end, from which the skin has been 

 removed. B, ventral view of the head, a.c, dotted line indicating position of auditory capsule ; 

 c.r, cartilage ring of nasal tube ; m, mouth ; m.s, mucus sac ; m.t, muscles to tentacles and lip ; my, 

 myotome ; n.ea, nasal capsule ; no, median nostril ; o.m, oblique muscles ; op, opening of mucus 

 sac ; sob, stiborbital branch, and ts, preorbital sensory branch of trigeminal nerve ; v.m, 

 ventral muscles ; 1, 2, 3, 4, tentacles. 



The adult Myxinoids differ strangely from the Lampreys and the 

 embryos of all other Craniates in that the afferent vessels pass to 

 the gill-sacs themselves, and not between successive pairs to the 

 gill-arches (Fig. 32). 



The myomeres give rise to dorso-lateral and ventral longitudinal 

 muscles, outside which extends a sheet of obliquely circular muscles 

 (Fig. 29). The myotomes alternate from side to side as in 

 Amphioxus. 



