98 



SUB-CLASS MARSIPOBRANCHII (CYCLOSTOMATA). 



The skeleton consists of cartilaginous, notocliordal, and mem- 

 branous tissue ; there is no bone. There are two kinds of 

 cartilage at least, the hard with considerable intercellular 

 matrix, and the soft with but little. 



The vertebral column consists of a persistent notochord with a 

 tough sheath, which is formed of two layers, an inner somewhat 

 fibrillated chordal sheath (or memhrana elastica interna) and an 

 outer thin elastic coat {memhrana elastica externa) ; both are 

 devoid of nuclei. The notochord so constituted is surrounded 

 by a nucleated membranous sheath (the so-called skeletogenous 

 tissue, or memhrana reuniens), which extends dorsalwards on 

 each side so as to enclose the spinal cord. Small cartilaginous 



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Fig. 50. — Cartilaginous skeleton of the anterior part of the body of Petromyzon ftuviatilis ; 

 side view (after A. Schneider). 1 Foramen for sensory, 2 for motor root of spinal nerve ; 

 3 eleventh dorsal arcualium ; 4 first '.dorsal arcualium, pierced by the first anterior root, 

 which passes into the ligament between the fourth and fifth myomere ; 5 foramen for 

 vagus ; 6 auditory capsule ; 7 foramen for trigeminal ; 8 foramen for optic ; 9 nasal 

 capsule ; 10 posterior dorsal cartilage ; 11 anterior dorsal cartilage ; 12 annular lip carti- 

 lage ; 13 anterior lateral cartilage ; 14 styliform cartilage of 12 ; 15 unpaired lingual 

 cartilage; 16 posterior lateral cartilage; 17 subocular arch; 18 styloid process; 19 

 cornual cartilage ; 20 branchial basket-work ; 21 the seventh gill aperture (the first and 

 sixth gill apertures are omitted) ; 22 pericardial cartilage. 



pieces are developed on the membrana reuniens on each side : 

 these, the dorsal arcualia {dorsalia), are roughly segmentally 

 arranged. 



In Petromyzon there are in the branchial and trunk region two pairs of 

 arcuaha in each segment, while in the tail they are fused to form on each 

 side a continuous ridge, with which the cartilaginovis fin-rays (somactids) 

 here present are continuous. Ventraha, fused to a continuous ridge, are 

 also present in the caudal region, and are continuous ventrally with ventral 

 somactids. In myxinoids cartilaginous elements are restricted to dorsaUa 

 and somactids in the caudal region, and to some somactids in the trunk. 



Anteriorly the notochord extends into the base of the skull, 

 ending just behind the pituitary body. 



The skull consists of cartilage and membrane. The roof is 

 entirely membranous in myxinoids, but in Petromyzon there is a 

 narrow bar of cartilage passing across the posterior part of the 



