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 off 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. 
Il. 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- 
hemal spines compose the thin compressed region just above the 
anal fin. These muscles, as well as the interhemal 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 dermohemal spine (anal ray). The 
internales arise from the dorsal portion of the interhemal spine 
and are inserted on the top of the dermohemal spine on each side of 
its articulation with the interhemal spine. The interhemal spine is 
a slim, straight bone, with its dorsal end pointed. On its ventral 
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 dermohemal spine has a cup-shaped articulation on its 
dorsal end which fits around these three heads of the interhamal 
spine in the nature of a ball and socket joint. The presence of 
the two small heads on the interhemal spine in the cephalo-caudal 
line allows the dermohemal spine only a limited motion in that 
direction, but a free movement laterally. The undulating move- 
Fic. 18. Anal ment of the anal fin results from the alternate contraction of the 
musculature. Hig- right internalis and externalis and then the left. Fig. 18 shows the 
(a be ae anal musculature of H. virescens on a large scale. The muscles 
nalis externalis; may be seen in Plate XIX, figs. 19, 20, 21, 22, 23, which are entire 
pai, pinnalisanalis eross-sections. In the cross-sections, however, the anal muscula- 
es ae hg ture is drawn in the same plane as the trunk musculature and the 
et. -ventro-caudal slope is disregarded, so that the entire muscle may 
dhs, dermohemal 
spine. be seen. 
