22 
THE YOYAGE OF H.M.S. CHALLENGER. 
outer lip. In front of this septum, and between both the lips, there exists a triangular 
groove deep in front (fig. 16) ; above the septum also there is a deep, nearly funnel-shaped 
cavity, the upper wall of which is formed from the anterior end of the under side of 
the substance of the mandible ; the inner margin is here much more prominent (fig. 16). 
Transverse sections of the jaws show the colour of the main portion to be a pale, clear, 
chitinous yellow, only a thin layer on the under side and adjoining part was of a chestnut 
brown. — The large mass of muscles (fig. 19 d) (5*5 mm. broad posteriorly), which lies 
between the hinder margin of the jaws, is made up of thick horizontal transverse fascicles, 
quite distinct from each other ; the function of these muscles is to divaricate the mandibles. 
The narrow mouth-cavity lies between the huge mandibles ; it increases slightly in 
width in its course backwards and downwards, where it is entirely filled up by the tongue 
(fig. 1 9 a). The tongue is strongly compressed ; it is short and keel-shaped, and 
has a length of 2 *25 mm., and a height of 2 mm.; the anterior and upper margin is 
traversed by a deep groove ; the muscular mass belonging to this tongue is also 
compressed, and is 3 '5 mm. in length by 3 mm. in height, and 2 mm. in thickness at the 
cleft hinder end, lying in front of the mandibular muscles. The broad radula extends 
beyond the end of . the tongue ; there are sixteen series of pale yellow-coloured 
teeth ; beneath the strong tectum radula (fig. 19 b), and within the radula sheath, which 
is dilated into an ampulla (fig. 19 c) posteriorly, are six developed and two not fully 
developed series ; the total number therefore is twenty-four. In the eleventh series 
there are thirty lateral teeth on either side of the median tooth ; within the foremost 
part of the sheath there are fifty-four, but the number does not appear to increase farther 
back. The plates were of a yellowish colour. The length of the lateral teeth of the radula 
reaches '28 mm., with a height of ’05 mm.; the length of the next outermost tooth is T2 
mm., while the outermost one measures only T mm. in length, the first being ‘03, and the 
second ’025 mm. in height. The length of the coloured portion of the median tooth 
is from *08 to T mm., by a breadth of ‘02 and a height about the same. The median 
teeth (PI. Y1II. fig. 20 a, PL IX. 7 a) are narrow, and only coloured in about their 
posterior third ; farther forwards they are rather higher ; the apex of the low, slightly 
bent hook reaches beyond the base (fig. 20 a). The lateral teeth also are narrow and 
slender (figs. 20 b, 7, 8), and somewhat bent; the basal portion is cut short in front, but 
rounded behind ; the hook short but well developed ; the yellow-coloured part of the 
teeth is continued anteriorly into a colourless cuticula (fig. 20 c, 7, 8). The innermost 
lateral tooth is the longest (figs. 20 b, 7 c). The length of the teeth diminishes gradually 
as we pass outwards, and somewhat more quickly in the outmost part (fig. 22, 8 a). 
Occasionally irregularities may be observed in the form of the teeth (fig. 21). 
The salivary glands are slightly separated from each other ; they are attached to the 
anterior margin and under surface of the stomach ; the colour is brownish-grey. The 
oesophagus (PI. VIII. fig. 15 c) is short. The stomach closely resembles that of the 
