PISCES. 



Skull of Sea Lamprey (Petromyzon marinus). After J. Midler. 



a, cartilaginous spinal axis ; b, c, rudimentary cartilaginous neural arches ; 

 d, d, part of cartilaginous respiratory framework ; g, auditory capsule ; h, cra- 

 nium j i, i", i , processes therefrom giving attachment to the muscles of the 

 tongue ; k, nasal capsule ; I, cartilage covering hinder part of mouth ; n, o, p, q, 

 anterior cartilages entering into composition of mouth j r, s, os hyoides. 



nous framework situated on the sides of the 

 neck, enclosing the branchial apparatus, and 

 allowing respiration to be accomplished by a 

 peculiar mechanism which will be described in 

 its proper place. This singular structure con- 

 sists of seven pairs of cartilaginous arches de- 

 rived from a kind of sternum situated in the 

 mesial line beneath the throat. These arches, 

 which have nothing in common with the bran- 

 chial arches of Fishes, mount upwards inter- 

 ruptedly towards the spine, giving off anterior 

 and posterior processes to form a kind of frame 

 to the branchial orifices. Anteriorly this re- 

 markable apparatus is attached to the cranium, 

 while posteriorly it terminates in a thin cartila- 

 ginous capsule, which encloses the heart; it 

 will, however, be better to describe it further on. 



In Ammocetes the skeleton is even more im- 

 perfect than in the Lamprey, all its parts remain- 

 ing permanently in a membranous condition, so 

 that they would seem to resemble worms more 

 than vertebrated animals, and in fact were ab- 

 solutely classed as worms even by the great 

 Linnaeus. 



Skeleton of Branch iostoma. In this re- 

 markable Fish the entire spine is made up of 

 a succession of very delicate membranous rings 

 without any apophyses whatever ; neither in 

 young specimens is the slightest trace of a 

 cranial dilatation of the vertebral column appa- 

 rent, probably owing to its being at this period 

 quite gelatinous in its texture, and conse- 

 quently translucent; and even in adults such 

 is its softness that it is impossible to distin- 

 guish it satisfactorily ; but along the back from 

 sixty to seventy vertebrae are easily counted, 

 the divisions between them being indicated by 

 slight bulgings and lines passing obliquely 

 from above downwards on the sides of the 

 column. In this way a separation between 

 the racliidian rings is rather indicated than 

 proved to exist; for, although there is, so to 

 express it, a tendency to divide at the points 

 indicated, the division is rather artificial than 

 natural. 



According to Mr. Goodsir, the chorda dor- 

 salis is formed externally of a fibrous sheath, 



967 



and internally of an 

 immense number of la- 

 minBe, each of the size 

 and shape of a section 

 of the column at the 

 place where it is situ- 

 ated. When any por- 

 tion of the column is 

 removed, these plates 

 may be pushed out of 

 their sheath like a pile 

 of coins. They have 

 no great adhesion to 

 one another, are of the 

 consistence of parch- 

 ment, and appear like 

 flattened bladders, as if 

 formed of two fibrous 

 membranes pressed to- 

 gether. 



Two ligaments may 

 be detected, one run- 

 ning along the upper, the other along the lower 

 aspect of the spinal column ; and from its 

 sides aponeurotic laminae pass off to form 

 septa of attachment between the layers of mus- 

 cles. Along the mesial plane above the co- 

 lumn a similar aponeurosis separates the supe- 

 rior lateral muscular masses, and by splitting 

 inferiorly, so as to join the sides of the rachi- 

 dian chord, forms the canal for the spinal 

 medulla. Foramina exist all along this canal 

 for the passage of the nerves. A similar apo- 

 neurotic septum is situated along the inferior 

 part of the column from the anal opening to 

 the extremity of the tail. Imbedded in these 

 two aponeuroses are cartilaginous rudiments 

 representing the superior and inferior vertebral 

 spines, but these are of extreme softness and 

 delicacy; and the traces which exist of the 

 transverse processes and ribs are in the same 

 soft condition, so that they are with difficulty 

 distinguishable. In Branchiostoma, therefore, 

 the locomotive skeleton may be said to consist 

 of the vertebral column only, without either 

 cranium or appended limbs. 



There is, however, an exceedingly elaborate 

 framework of soft cartilaginous arches which 

 surrounds the branchial chamber, and forms 

 a kind of branchial thorax, the nature of which 

 will be examined further on. 



Arthrodial system. The articulations of the 

 bones of Fishes present the same varieties as 

 those of other animals; only the arthrodial 

 and ginglymoid are more rarely met with 

 because their limbs have not to execute such 

 complicated movements. 



It is by means of a ginglymus that the 

 lower jaw and operculum are attached to the 

 pterygo-palatine apparatus, and the latter to the 

 cranium. The same articulation occurs be- 

 tween the rays of the dorsal and anal fins with 

 the interspinous bones, and between the first 

 ray of the pectoral fin with the bone analogous 

 to the radius. 



There are, moreover, in Fishes two kinds of 

 articulations having determinate movements 

 which are not met with in the other classes : 

 one is formed by two rings joined one to the 



