728 MR FRANK J. COLE 



This very remarkable and unique muscle varies to such an extent, at times even in 

 its general arrangement, that no typical description is possible. Hence the present 

 account has not been generalised, but details the conditions as found in a perfect 

 specimen fixed in formol-alcohol. The result of this fixative is to whiten the tissues 

 generally, and, so far, to keep them white. Hence the present muscle, always very 

 difficult to dissect, is too white for accurate investigation. This, however, is easily 

 remedied by soaking the specimen in very dilute chromic acid, which colours the 

 muscles first yellow and then brown. A number of individuals were dissected, and the 

 more important variations are recorded in the following description. 



When a lateral incision is made through the body-wall at the region of the club- 

 shaped muscle and gills and the flaps pinned back, it is seen that the space between 

 the notochord and these structures is occupied by a quantity of adipose tissue. When 

 this and the vagus nerve are removed from the roof of the club-shaped muscle, the 

 oblique fibres of the constrictor pharyngis are exposed. On turning now to the gill 

 region, the four anterior gills are seen, more or less faintly, showing through a layer of 

 adipose tissue, which is deposited most densely in the depressions between the gills and 

 between their ducts, so as to render the surface of the branchial region quite level. On 

 removing this tissue, partly with a fine needle and partly by brushing it away, we see 

 the whole of the first gill and decreasing portions of the next four lying in their gill 

 sacks, which are usually more or less filled with blood.* The last gill and the greater 

 part of the fifth are covered by a thin sheet of muscle, the fibres of which (cp. especially 

 fig. 13, c. b. c.) course downwards and sharply forwards, becoming more and more 

 horizontal towards the posterior end of the muscle, until the fibres at the ventro- 

 posterior region are quite horizontal. 



In some specimens this sheet of muscle was so reduced as to be practically absent. 

 It is not present in Bdellostoma according to J. Muller, where only the loops are 

 visible over the gill sacks. 



At the posterior margin of the sheet there were a few fibres passing almost vertically 

 downwards but slightly backwards on to the posterior surface of the ductus oesophago- 

 cutaneus (fig. 13, 11, and d. ces. ct.). The sheet meets the corresponding one of the 

 other side at a somewhat irregular linea situated roughly in the mid-dorsal line. In 

 Bdellostoma, according to J. Muller, there is an incomplete longitudinal fissure in the 

 middle line, and the somatic arteries emerge at the lacunae. He further states elsewhere 

 that there is a crossing of the fibres at the mid-dorsal line in Bdellostoma and Myxine, 

 so that the portion " was nach hinten auf der rechten Seite herabsteigt, ist die Fortsetzung 

 der Fascikel, die vorn links heraufsteigen und umgekehrt" (p. 272). I have not seen 

 any trace of this in Myxine. 



The systemic aorta (s. ao.) passes underneath the two opposed sheets, i.e. between 

 them and the gut. The somatic arteries, therefore, have to perforate the muscle in 

 order to reach the back (cp. figs. 3, 13). In the specimen now described three such 



* Cp. Cole, 5, p. 325. 



