708 MR FRANK J. COLE 



a description of the protractor muscles, which is noticed elsewhere. In 1894 Ayers* 

 extended the observation to Bdellostoma, and gives two figures of this genus, showing 

 the teeth out, but no description of the protractor muscles. Finally Goode and Bean t 

 in 1895 figure, but do not mention, this phenomenon in Myxine australis. 



The function of the two protractor muscles has, as before stated, been already 

 discussed in some detail both by J. Muller and P. Furbringer, but their results are 

 unsatisfactory for two reasons : (l) they failed to elucidate the anatomy of the super- 

 ficial protractor muscle (copulo-glossus superficialis) ; (2) they did not realise that a dental 

 apparatus derived from and attached to the mucosa cannot move without carrying; the 

 mucous membrane with it, i.e. the teeth cannot be everted without the mucosa of the 

 floor of the mouth being also evaginated. This oversight of the latter point is 

 evidently due to the fact that they worked too much with dissected specimens. If a 

 vertical longitudinal incision be made through the roof of the head and the two halves 

 pinned out on each side so as to expose the mouth cavity, it is seen that when the dental 

 apparatus is at rest (i.e. withdrawn) there is an area of puckered mucous membrane 

 immediately behind it, whilst the mucosa in front of it is more or less tense. Now. if 

 a similar section be made with the dental plates everted, it is seen that the mucosa on 

 the floor of the mouth is on the stretch, whilst externally and immediately behind the 

 (now) ventral margin of the ventral plates there is an area of evaginated and folded 

 mucous membrane. It therefore follows that immediately behind the " tongue ' : there 

 is a special fold of loose mucosa to admit of the forward movement of the teeth, and 

 also that there is probably some muscular apparatus for evaginating the mucosa itself 

 in front of the teeth. 



The function of the deep protractor (copulo-glossus profundus) is quite simple. It 

 draws the "tongue" straight forwards in the mid-ventral line, but we may deduce 

 from its attachment to what is a ventral skeleton that it would probably require 

 assistance before it could topple the dental apparatus over the ventral margin of the 

 mouth, and so on to the exterior, — i.e. it is adapted to produce motion in a horizontal, 

 but not in a vertical plane. This assistance is admirably provided for by the super- 

 ficial protractor, which first of all evaginates the anterior ventral mucosa (thus not only 

 paving the way, but indirectly reinforcing the pull of the deep protractor), and finally, 

 when the " tongue " has arrived at the anterior extremity of the mouth, exercises a sharp 

 ventral erecting pull (by its dorsal attachment) on the teeth themselves, in this way 

 supplying the jerk necessary to tumble the whole apparatus downwards out of 

 the mouth. P. Furbringer states that the tether of the hyo-copulo-glossus assists 

 in this final act, as described under that muscle. The two protractor muscles, 

 therefore, though playing somewhat different parts, perform one function between 

 them, instead of both acting in much the same way, as assumed by J. Muller and 

 P. Furrbinger. 



* Biological Lectures at Wood's Holl, ii. pp. 136-7, 141-2, 1893-4. 

 t "Oceanic Ichthyology,''' Atlas, pi. i. fig. 2. 



