160 



MEMOIRS OF THE CARNEGIE MUSEUM. 



While experimenting with these fishes the caudal appendage of several was 

 removed. This seemed in no way to influence the speed or method of swimming. 

 When a large portion of the anal fin was cut ofT the fish swam by means of the 

 pectorals. If the pectorals alone were removed, the fish swam by the use of the 

 anal fin and body motion together. This was probably due to an effort to guide 

 itself, since the guiding is done almost entirely by the pectorals. The anal could 

 be used for either backward or forward movement. 



II. Anal Musculature. 



The muscles moving the anal fin, the muscles pinnalis analis externalis and 

 pinnalis analis internalis, together with the muscles lateralis imus and the inter- 

 hosmal spines compose the thin compressed region just above the 

 anal fin. These muscles, as well as the interhcemal spines, are 

 directed ventro-caudal at an angle of five to ten degrees to the long 

 axis of the fish, hence in a cross-section of the body the obliquely cut 

 ends of several show in the anal region. There is a pair of pinnales 

 analis externalis and pinnales analis internalis for every anal ray. 

 The externales are the larger of the two. These muscles have 

 their origin in the skin on each side and their insertion on a lat- 

 eral process on each side of the dei'mohcemal spine (anal ray). The 

 internales arise from the dorsal portion of the interhcemal spine 

 and are inserted on the top of the dermohcemal spine on each side of 

 its articulation with the interha'mal spine. The interhcemal spine is 

 a slim, straight bone, with its dorsal end pointed. On its ventral 



,dlis end it bears an enlarged rounded head, and two smaller knobs a 

 little lower down. These knobs lie in the median cephalo-caudal 

 line. The dermohcemal spine has a cup-shaped articulation on its 

 dorsal end which fits around these three heads of the interhcemal 

 spine in the nature of a ball and socket joint. The presence of 

 the two small heads on the interhcemal spine in the cephalo-caudal 

 line allows the dermohcemal spine only a limited motion in that 



I direction, but a free movement laterally. The undulating move- 



ment of the anal fin results from the alternate contraction of the 

 right internalis and externalis and then the left. Fig. 18 shows the 

 anal musculature of E. virescens on a large scale. The muscles 

 may be seen in Plate XIX, figs. 19, 20, 21, 22, 23, which are entire 

 cross-sections. In the cross-sections, however, the anal muscula- 

 ture is drawn in the same plane as the trunk musculature and the 

 ventro-caudal slope is disregarded, so that the entire muscle may 

 be seen. 



Fig. 18. Anal 

 musculature. Eig- 

 enmannia virescens 

 (Val.) pae, pin- 

 nalis externalis; 

 pai, pinnalis analis 

 internalis; ihs, in- 

 teih;rmal spine; 

 dlis, clermoha'mal 

 spine. 



