OX THE GENERAL MORPHOLOGY OF THE MYXINOID FISHES. 709 



The " Club-Shaped Muscle." 



This large muscle mass, by far the most striking feature in the myology of the 

 Myxinoids, and the development of which is by some believed to be the cause of the 

 suppression of the anterior gills and the displacement of the functional gills, is 

 composed of three discrete muscles, which are, however, enclosed within a tough 

 external fascia to form the so-called " club-shaped muscle," occupying a ventro- 

 longitudinal position, with the handle projecting straight forwards. The principal 

 attachments of the complex are in front, where it is connected with the posterior 

 segment of the basal plate and with the dental apparatus. It has also two slight 

 attachments to the lateral head of the copulo-glossus profundus and the copulo- 

 quadratus superficialis. Dorsally it is slung to the parietal muscle by the second and 

 third divisions of the constrictor pharyngis, whilst behind it may receive the first loop 

 of the constrictor branchiarum et cardise, and also a slight bundle from the ventral 

 longitudinal tract of the same muscle, as shown in fig. 3. Posteriorly, the first pair 

 of gill sacks overlap the club-shaped muscle dorso-laterally. 



The whole mass, together with the posterior segment of the basal plate and 

 the dental apparatus, may be easily removed from the body and studied in detail. 

 Portions of all three muscles constituting it may then be seen without further 

 dissection. 



18. M. copulo-copularis. (Figs. 2, 3, cp. c.) 



J. MOller, Hohle aussere Muskel der Zunge (p. 254). 



,, Mushulbse Capsel [Scheide] des Ldngswuskds der Zunge (p. 315). 



Ayers and Jackson, M. constrictor musculi mandibuli (p. 205). 



A distinctly paired (as observed by Schneider) and very powerful transverse 

 constrictor muscle, with an extreme length of 38 mm., width 11 mm., and depth 

 7 mm., in a 35 cm. Hag. It is pointed in front and obliquely truncated behind, and 

 the longitudinalis linguae projects behind its posterior border. Its dorsal surface is 

 flat and its ventral surface convex. The anterior extremities of the muscle are very 

 narrow, and are situated immediately between the dorsal margins of the limbs of the 

 U-shaped posterior segment of the basal plate so as to complete the channel dorsally. 

 They terminate slightly in front of the greatest depth of the cartilage, i.e. just anterior 

 to the commencement of the taper at the posterior end of the anterior fourth. The 

 muscle, however, soon begins to increase both in width and depth — as regards the 

 latter, the increase corresponding to the diminution or dorsal taper of the posterior 

 segment of the basal plate, until the fibres appear to meet behind the point of the 

 cartilage at the mid-ventral line. In other words, the pointed extremity of the 

 cartilage apparently interrupts the course of the fibres at the mid-ventral region. In 



