CYCLOSTOMATA 



89 



The myotomic musculature, the median fins, the digestive, circulatory, 

 and excretory system are much as in the fishes. 



A more special account of the characters of cj^clostomes will best be 

 presented when the two sub-classes Myxinoidea and Petromyzontia 

 are compared. 



Sub-Class I. Myxinoidea 



These "hag-fishes" or "borers" are called myxinoids because of 

 their habit of producing, when captured, great quantities of slimy 



;? 



W£ 



c 



Pig. 47. — Myxine. A, External view of entire animal; the rows of pores are 

 openings of mucous glands; no eyes. B. Ventral view of anterior end, showing 

 terminal nostril, oral hood with buccal tentacles; ha, single pair of branchial aper- 

 tures (cf. D); mg, openings of mucous glands. C inner ear showing single semi- 

 circular canal. D. Internal anatomy, ac, auditory capsule; 6.s, branchial sacs, 

 opening by means of a common branchial tube (chl) into the common branchial 

 aperture (6a); g, gut; h, heart; ihc, notochord; rd, nasal tube; sc, spinal chord; 

 tc, tongue cartilages; tm, tongue musculature. (Redrawn after Parker and Haswell.) 



mucous jelly. It is said that one large specimen will make a bucket- 

 full of jelly. Hags lead a quasi-parasitic life, in that they commonly 

 enter the gills or mouths of dead or disabled fi.shes and remain inside 

 till they have so completely gutted the quasi-host that only the shell 



