584 



PHYLUM CHORDATA : CLASS CYCLOSTOMATA 



of the ectodermal infolding remain in contact instead of forming an open 

 medullary canal. 



In the head region, where the gut is not surrounded by yolk-cells, 

 the mesoderm is formed from hollow folds in " enterocoslic " fashion ; 

 but in the trunk region the cushions of endodermal yolk-cells change 

 gradually into mesoderm, and acquire a cijelom cavity in " schizocoelic " 

 fashion. Thus the two main ways in which a body cavity arises — 

 (a) from coelom pouches of the archenteron, (b) from a splitting of solid 

 mesodermal rudiments — are here combined. 



The larva, or Ammocoetes, is an Amphioxus-like creature without the 

 sucking mouth, tongue, or teeth of the adult (see Fig. 327). The 

 head is small, with a hood-like upper lip and separate lower lip. There 

 is a velum at the top of the pharynx, ciliated bands, and an endostyle. 

 Seven pairs of incipient gill-pouches open directly from the pharynx 

 to the exterior. Pharynx and oesophagus form a continuous tube. 

 The eyes are half made and hidden beneath the skin. The delicate 

 dorsal fin is continuous. 



Contrast between Hag and Lamprey 



Hag (Myxine). 



Excliiiiively marine. 



The fill is confined to the tail. 



Numerous large glands in the com- 

 plex, slimy skin. 



Mouth with barbules, no lips, few 

 teeth. 



Skull without any roof. 



Skeletal system less developed than 

 in the lamprey. Only a hint of a 

 branchial basket. 



Cerebrum and cerebellum rudiment- 

 ary. 



Eyes hidden and rudimentary. 



Ear with one semicircular canal. 

 Nasal sac opens posteriorly into the 

 mouth cavity. 



Six pairs of gill-pouches, opening 

 directly into the gullet, less directly to 

 the exterior. 



Longitudinal ridges in the intestine. 



No urinogenital sinus ; one genital 

 pore. 



Ova large and oval, with attaching 

 threads ; meroblastic in Bdellostoma. 



Development unknown in Myxine ; 

 direct in Bdellostoma. 



Lamprey (Pelromyzon). 



In rivers and seas. 



Two unpaired dorsal fins. 



Sensory structures in the complex, 

 slimy, pigmented skin. 



No barbules (except in the larva), 

 but lips, and many teeth. 



Skull very inaperfectly roofed. 



Hints of vertebral arches. 



Cartilaginous basket - work around 

 gill-pouches. 



All the usual parts of the brain are 

 distinct. 



Eyes hidden and retarded in the 

 larva, exposed and complete in adult. 



Ear with two semicircular cauals. 



Nasal sac ends blindly. 



Seven pairs of gill-pouches, opening 

 directly to the exterior, less directly 

 into the adult gullet. 



A slight spiral fold in the intestine. 



A urinogenital sinus, and two genital 

 pores. 



Ova small and spherical ; holo- 

 blastic. 



Development with metamorphosis. 



