740 MR FRANK J. COLE 



pass through the entire thickness of the muscle, and hence the length of the muscle 

 fibres, which course more or less directly longitudinally from one septum to another, is 

 confined to the intervals between the septa. Hence, as Wikstrom says (p. 406) : " Ein 

 jedes Transversal-septum fungirt also zugleich als Ursprungssehne fur das eine Myomer 

 (Muskelbauch) und als Endsehne fur das andere, angrenzende." Further, the fibres on 

 the two sides of any one septum are often placed exactly opposite each other, so as to 

 be directly continuous except for the septum. 



The course of the septa through the thickness of the parietalis, i.e. from the fascia 

 superficialis externa to the f. s. interna, can be studied either by a series of longitudinal 

 radial hand sections, or, and much better, by picking and brushing away the muscle 

 fibres from a macerated specimen. It is then seen that the septa do not everywhere 

 pass straight through the width of the muscle, i.e. at right angles to the long axis 

 of the body, but follow largely an oblique course, as shown in fig. 14. A typical 

 septum from the middle of the body may be divided into four parts, commencing at 

 the back, as follows (cp. the figure) : — 1. Externally commences near the middle line, 

 and passes obliquely downwards and forwards at a sharp angle. It penetrates the 

 muscle almost straight downwards, i.e. vertically, at right angles to the surface at this 

 region, but somewhat inwards, and is attached internally at the middle of its length to 

 the skeletogenous layer of the neural tube, near the junction of the latter with the 

 chorda. Both behind, and in front where it bends sharply on itself opposite the 

 junction above and turns backwards, it does not reach the neural tube or chorda, and 

 therefore has no internal attachment at these places. 2. As seen externally, passes 

 sharply backwards and downwards. It penetrates in a curve with the convexity external 

 and anterior, almost straight through the muscle, i.e. sub-horizontally, at right angles 

 to its external surface, but somewhat inwards, to become obliquely attached to the 

 skeletogenous layer of the lateral wall of the chorda. 3. Externally, passes almost 

 straight downwards, but slightly forwards, and courses obliquely through the muscle in 

 a forwardly-directed curve, with the concavity external and anterior, to become attached 

 to the fascia superficialis interna. 4. Continues the external course of 3, and passes 

 practically straight through the muscle, but slightly forwards, to become attached as 3. 

 As the muscle is thinnest at this region, the septum is here very narrow. In the 

 Lamprey, according to J. Muller, the septa are much more oblique, so that in a trans- 

 verse section one sees a greater number of them than in Myxime. 



I have counted the number of the myotomes in the different regions of the body in 

 six Hags taken at random, with the following results (see table, next page). 



J. Muller gives 109 as the number of myotomes in Bdellostoma. 



I have found the first few myotomes to vary somewhat. For example, the first may 

 be quite small and quadrangular, and situated immediately below the ventral border of 

 the tentacularis posterior, or, as is generally the case, its dorso-posterior angle may be 

 prolonged upwards and backwards under the ventral edge of the above muscle, in some 

 cases as far as its posterior extremity, as shown in fig. 9. In these cases the normal 



