2 1 - 



ANATOMY OF VERTEBRATES. 



138 



Slu iitn- muscles arise, above, from the cranial cartilage, fig. 24, d, 

 and below, from the hyoid cartilage, to act upon parts of the sucker ; 

 the latter, g, It, diverge to their insertions. Part of the deep- 

 seated longitudinal expansor oris, more directly antagonising the 

 circular sphincter oris, a, is seen at ///, fig. 136. 



Details of the myology of the Myxinoids with a comparison of 

 the muscular system of Fishes with that of higher Vertebrates, will 

 be found in xxi. pp. 179-249. 



In the Trunk-fish (Ostracion) flexion of the trunk is abrogated 



by the case of ganoid armour, fig. 1 6, 

 dn, dh, inclosing the body, and which 

 leaves only the jaws and fins free. 

 The myocommas are accordingly re- 

 duced to a thin layer of longitudinal 

 fibres, modified posteriorly for inser- 

 tion into the moveable part of the 

 tail and its fin. 



In another plectognath, the odd- 

 shaped Sun-fish (Orthagoriscus) the 

 muscles of the continuous vertical fins 

 take the place of the ordinary myocom- 

 mas : those of the lofty dorsal com- 

 mencing behind the occiput ; those of the deep anal behind the 

 short abdomen : the dermoneurales arise from the integument, 

 especially the fibrous septum of the lateral line ; the deeper-seated 

 intemeurales from the neural and interneural spines. Each series 

 is more or less blended together, conformably Avith the degree of 

 confluence of the interneurals, upon the expanded ends of which 

 the spines of the dorsal fin move as one body, the anal fin having 

 a similar structure. Nevertheless, towards their insertion, the 

 fasciculi of the fin-rays become, like them, distinct ; each one 

 behind being sheathed by the one in front, and their long tendons 

 passing through lubricated grooves or sheaths to their insertions. 

 On the sides of the abdomen the muscles are reduced to two fas- 

 ciculi, expanding, the one from the clavicle, the other from the 

 coracoid, upon the peritoneum. 1 



Amongst the Plagiostomi, the Sharks are the most active and 

 powerful, and in them the muscular system is most developed, and 

 in certain parts most specialised. The more acute angles formed 

 by the intermyocommal septa have already been noticed, fig. 132. 

 A fasciculus continued from the upper portion is inserted, by a 

 strong aponeurosis into the upper part of the cranium, ib. a, a. 



Muscles of head and sucker; Lamprey, 



XLIII. 



1 xlvi. and cxcyii. 



