752 MR FRANK J. COLE 



angle formed by the junction of the latter with the median ventral bar of the caudal 

 fin skeleton. This agrees with what I find in a series of sections I have made of the 

 entire tail region, but in one dissection the dorsal fibres arose from the dorsal margin of 

 the median ventral bar, and certainly did not reach the chorda. This is the condition 

 figured ; (b) posteriorly it has a diffuse origin from the entire width of the ventral bar 

 — of course behind the region of the caudal heart ; (c) the ventral fibres have a long 

 origin from the lower margin of the ventral bar and from the roots of the anterior " fin 

 rays " fusing with the bar — in one specimen from eleven of these, in the Hag figured 

 from seven, but in the sections from two only, the heart and large vessels intervening 

 between the muscle and the " fin rays " in front. This part of the origin is not mentioned 

 by Eetzius in Myxine, but it is present in Bdellostoma, according to Greene. 



From their origin the fibres rapidly concentrate, so that the muscle now passes 

 straight forwards as a sharply converging belly, to terminate in a narrow insertion 

 (immediately behind the last slime sack) into the external margin of the tranversely 

 expanded extremity of the knob projecting from the anterior margin of the median 

 ventral bar (cp. Part I., fig. 17). The entire length of the muscle in a 30 cm. Hag was 

 7 mm. and its greatest width 3 mm. In Bdellostoma, according to Greene, the 

 measurements were 6'3 mm. and 2'8 mm. As Bdellostoma is, of course, a much larger 

 form than Myxine, it would seem that the muscle is not so well developed in the 

 former genus. 



The cordis caudalis muscles are crescent-shaped in transverse section, fitting over 

 and passing external to the paired caudal hearts, each of which lies between one of the 

 muscles and the median ventral bar. The caudal hearts possess no intrinsic mus- 

 culature, but have only fibrous walls, as pointed out by G. Retzius and Greene. In 

 this respect the caudal heart differs from the portal heart, which is controlled by 

 intrinsic musculature. 



The fibres of the cordis caudalis are in marked contrast to those of the adjacent 

 parietalis. They are striped, but have a very small calibre. This is not mentioned 

 by Greene. I refer to the character of these fibres in another part of this paper on 

 the histology of the muscles. 



The caudal hearts and their muscles were first discovered by G. Retzius, who 

 briefly described them. I shall deal with the function of the muscle when I treat of 

 the vascular system, but in the meantime I may quote the following statement by 

 Retzius, based on observations of the living animal : " Bei jeder ' Systole' kehrte sich 

 der Knopf der Knorpelplatte [the knob of the median ventral bar above] von links nach 

 rechts, und dabei entleerten die beiden paarigen Hertzsacke ihren Inhalt in die Caudal- 

 vene. Dann kehrte der Knopf wieder nach links zuriick und blieb einen Moment 

 stille ; so kam eine neue Verschiebung nach rechts mit einer Ausleerung der Siicke u. s. w. 

 Dabei waren deutlich die beiden seitlichen platten Muskeln die Motoren, welche 

 gleichzeitig auf die Wande der Hertzsacke driicken und den Inhalt vorwiirts verschieben." 

 This account differs from that given by Greene of Bdellostoma, and it is quite 



