SCAI.E OBSERVATIONS OF THE SQUETEAGUE AND PIGFISH. 29I 



wall of the scale pocket unite themselves at the anterior border of the scale with the deepest layer of the 

 dermis in which the fibers have a course parallel with the sxu^ace of the body. The imier wall of the 

 scale pocket at its posterior part unites with the outer wall of the adjoining posterior pocket. Farther 

 forward it is built up of the fibrous processes of the deep epidermal layer. Near the scale its condi- 

 tion changes, as immediately toward the inside the same number of cells is fotmd in a ground substance 

 only slightly developed and not fibrillated. The fibers of the deep dermis layer have a similar arrange- 

 ment to that of the ganoids and selachians." 



FORM AND MODE OF ORIENTATION.- 



Ryder (1893) worked on the arrangement of the scales on the body, seeking to 

 account for their arrangement in rows and their imbrication. He shows that scales 

 may lie in rows in three directions: (i) Downward and backward; (2) downward and 

 forward; (3) along the long axis of the body. 



He advances a most interesting opinion in explanation of this method of orienta- 

 tion, viz., that it is due to the segmentally arranged muscles of the body. In support 

 of this he notices that in archaic types the number of scales corresponds with the num- 

 ber of somites in the body. He summarizes two important conclusions: 



1. Scales of fishes bear a segmental relation to the remaining hard and soft parts of the body and are 

 either repeated consecutively in oblique rows corresponding to tlie number of segments, or they may be 

 repeated in rows corresponding to the number of somites, or segmental reduction may occur which 

 may aSect the arrangement of the scales so as to reduce the number of rows below the number of somites 

 indicated by the other hard and soft parts. 



2. Tlie peculiar manner of interdigitation of the muscular somites as indicated by the sigmoid 

 outline of the myocommata as seen from tlieir outer faces and the oblique direction of the membrane 

 separating tlie muscular cones has developed a mode of insertion of the myocommata upon the corium 

 which has thrown the integument into rhombic areolae during muscular contraction. These areolae 

 are in line in three directions and the folds separating them, particularly at their posterior borders, 

 are inflected in such a manner by muscular tensions due to the arrangements of the muscular cones as 

 to induce the condition of imbrication so characteristic of the squamation of many fishes. 



Ryder seems to be the only scale investigator who takes into consideration the 

 adaptation of the stiff scale to the movements of the fish's body — a subject which will 

 be considered in connection with the function of the radii in another part of this paper. 



Under the caption "Form and mode of orientation" in his paper, Baudelot (1873) 

 takes notice only of the extreme variability, from genus to genus, from species to 

 species, between individuals and even in the same individual. It is a well-known fact 

 among modem taxonomists that the number of scales in longitudinal rows is constant 

 enough within certain limits to be a valuable taxonomic character. 



SIZE. 



The basis of age determinations is the fact that the scales are constant throughout 

 life, both in identity and number. Steenstrup (1861) noted that cycloid, ctenoid, and 

 ganoid scales grow throughout life and increase in size proportionately to that of the 

 fish, while placoid scales never exceed certain limits, but fall off, giving rise to others. 

 The size and shape are agreed upon as constant within certain limits, and Cockerell 

 and others use them as taxonomic characters. It is understood, however, that size 

 is by no means constant. 



o Thomson's translation. 



