56 



BULLETIN OF THE BUREAU OF FISHERIES. 



Their relations are shown by a 



corresponding to the skeletal divisions of the fin itself, 

 consideration of the anal fin skeletal complex. 



SKELETON OF THE ANAL FIN. 



The anal fin of the king salmon consists of i6 well-developed fin rays, with three 

 rudimentary rays at the anterior margin. There are no spines. A pair of typical rays, 

 say from the middle of the series, serves to show the general skeletal plan (fig. 1 1). 



The ray itself consists of two half rays, the so-called dermal plates, intimately bound 

 to each other except at the base. Near the base the plates diverge slightly and curve 

 sharply posteriorly, not unlike the curve of a hockey stick. The ray is seated cross- 

 saddle fashion over a median cartilage to which it is strongly 

 bound by connective tissue ligaments but with a movable joint. 

 The median cartilage is bound by a movable joint to the 

 head of the supporting interhemal. The interhemal is firmly 

 imbedded in the median longitudinal septum. The position of 

 the interhemals alternates with the hemals but forms an acute 

 angle with the axis of the body, the inclinations being directed 

 caudally. 



This skeleton is modified at two points. Anteriorly the 

 whole complex just described is represented by a triangular 

 plate, apparently the homologue of either the median cartilage 

 or more probably of the interhemal. This plate is strongly 

 V " bound to the anterior margin of the fin base. Its most dorsal 



angles receive the tendons of the retractor ischii (fig. 2). 



Posteriorly there is also a sharp skeletal modification. The 

 last two interhemal spines at the posterior end of the series are 

 fused at their ventral ends forming an irregular club-shaped knob 

 under the second from the last fin ray. This interhemal knob 

 is larger and stronger than the ones immediately in front of it. 

 Just dorsal to the last rays of the anal fin, and in series 



C.C. 



-o. 



Flc. II. — Two segments of the 

 skeleton of the anal fin. The 

 lettering is on the cephalic bor- 

 der; r., ray; int., interhemal 



spine; C.C. cartilages. o, ossicle ^\\\^ and bound by the enlarged interhemal, is a specially mod- 

 forming movable joints be- .^, -i • i t- \ 



tween the interhemals and the ihed cartilagmous plate. It IS rather strong and laterally com- 



fi°">'5- pressed. This plate receives the attachment of the retractor 



analis. The plate is very strongly bound to the ones in front of it and to the inter- 



spinous septum by bands of fibrous connective tissue. Doubtless this modified cartilage 



is the homologue of one or more intermediate plates or of the interhemals. 



INCLINATOR ANALIS. 



The constituent serial divisions of this muscle are exposed by removing the skin 

 from along the base of the anal fin and the ventral surface of the adjacent part of the 

 body (fig. 13). There are muscle divisions for each ray including the rudimentary rays 

 at the anterior margin of the fin. The largest and longest divisions are opposite the 

 anterior full rays of the fin. The muscle slips become progressively smaller posteriorly. 

 The muscles of the rudimentary rays are small and imperfectly separated. 



