18 



from the lower surface of the fin and, passing downward, is inserted partly on 

 the side of the mantle and partly upon the opposite fin. 



The fin (text figure 8) is formed by three sets of muscle fibres , longitudinal , 

 transverse and vertical , enclosed between three fasciae. Two fasciae are 

 subcutaneous , forming the surfaces of the fin , and the third lies at the middle 

 of the fin. The longitudinal muscle fibres form two thin sheets which lie upon 

 the upper and lower surfaces of the middle fascia. They extend from one to 

 the other free edge of the fin. The transverse fibres arise from the obliquely 

 inclined upper surface of the cartilage and, passing outward beneath the 

 superficial fasciae , are inserted in them and among the muscles. These fibres 



j are separated into long , strong and laterally compressed 



';'''"''-' :''''; , bundles by the short, vertical muscle fibres which, extending 

 ">! U' : .i' l ? r '" from the superficial fasciae to the middle fascia, form wall- 



i-'o-t -i-iy' i -_'lll.j like septa that extend from the base to the free edges of the 

 '% i s fin. The bundles of transverse fibres become more slender as 



:}■&.■?'[:' ; they approach the free edge of the fin. This is due partly 



v to the loss of fibres by insertion and partly to the branching 



Fig. 8. f the bundles. This reduction of the muscle bundles as 



they approach the free edge of the fin is partly counterbalanced by the union of 

 bundles with one another. The septa of vertical fibres become thinner, less 

 high , and more numerous near the free edge. Each septum is from : / 3 to 1 / i 

 as thick as the bundles which it separates. As its fibres approach the middle 

 fascia they diverge to pass between the longitudinal muscle-fibres, and hence the 

 septum seems broader near the fascia. These septa appear through the trans- 

 parent skin of the lower side of the fin as dark lines. The longitudinal , trans- 

 verse, and vertical fibres together form a strong flexible fin which is able to 

 make strong upward or downward strokes that instantly change the direction 

 of the squid's motion and also is capable of a slow undulating movement that 

 propels the squid slowly forward or backward. 



