CHIEF CHARACTERS 



structure, yet their chief diagnostic characters may be summarised 



as follows : The dermal 

 .dcv. skeleton is in the form of 



denticles and scales. There 

 are median fins supported 

 by endoskeletal radials (som- 

 actidia) ; and paired limbs 

 of similar structure. All 

 these limbs are fins adapted 

 to aquatic life, and have 

 the web strengthened with 



Transverse section of the trunk of Mnstelus 

 I'ntiif-tii-uf, Gthr. (after T. J. Parker), showing the 

 veins, i; abdominal coelorn ; cv, posterior car- 

 dinal; <?.((, dorsal aorta; d.c.v, dorsal cutaneous 

 vein ; d.i.v, dorsal intestinal vein ; i.i.v, internal 

 intestinal vein at edge of spiral valve ; l.c.v, 

 lateral cutaneous vein; l.v, lateral vein; m.v, 

 elonic vin. 



H 



FIG. 75. 



Embryo of Gdbius. (After Wencke- 

 bach, from Hertwig's Handbuch.) A, 

 aorta ; H, heart ; V.c.a, vena cardinalis 

 anterior ; V.si, vena subintestinalis ; D.C, 

 cluctus Cuvieri. 



dermal fin-rays (dermo- 

 trichia). An elaborate 

 system of lateral -line 

 organs extends over the 

 head and trunk. The 

 heart retains an un- 

 divided atrium, and 

 pumps venous blood into 

 the branchial lamellae 

 of the gill-arches. Not 

 more than eight gill-slits 

 are pierced, of which the 

 first is the relatively 

 small spiracle. As char- 

 acters which are con- 

 sidered primitive, but 

 are often lost through 

 specialisation, may be 

 mentioned : an uncon- 

 stricted notochord, a 

 diphycercal tail, a large 

 number of radials in the 

 median and paired fins, an amphistylic attachment of the jaws, a large 



FIG. 76. 



Torpedo embryo with yolk-sac. (After Riickert, from 

 Hertwig's Handbuch.) 1, vitelline artery; 2, vitelline 

 vein ; 3, remains of the peripheral sinus. 



