Ra a ae ee Se, ys 
( 
the Mouth in Sucking Crustacea. 17 
It remains to examine the structure of the bottom of the 
sucking-tube—that is, the region about the real mouth of the 
animal. We then find that the sphenoid plate reaches as far 
forward as the apex of the second joint of the palpi of the 
maxillipeds; here the ridge, already mentioned as running 
along the middle of the plate in question, bifurcates into two 
soft branches, lying close together, which coalesce with the 
lobes of the tongue, forming their thick inner margins, and 
presenting, with the open slit between them, the exact figure 
of a buttonhole. The two round, naked, soft, and slightly 
folded lobes of the tongue, which resemble flat cushions, fill 
entirely the small space between the sphenoid plate behind, the 
palpi of the maxillipeds on the sides, and the labrum in front, 
whilst their abruptly attenuated, short and conic ends are turned 
downwards and slung round the fore parts of the mandibles. 
The sucking-tube would thus be completely closed at the 
bottom, if the just-mentioned buttonhole-like slit between the 
lobes of the tongue did not remain; and this slit must there- 
fore be considered the real sucking-orifice. 
We have thus traced the ways in which all the necessary 
conditions for the hooking on and loosening, the contraction and 
extension, abbreviation and prolongation of the mouth-tube are 
supplied and combined into one collective whole, and we can 
now pretty well understand how this animal is enabled to suck. 
‘The mouth-tube itself is so constructed that it may serve as a 
sucking-cup; further in we meet the tongue with its button- 
hole and nothing more, therefore, is required than swallow- 
movements of sufficient strength to cause a liquid to ascend 
into the cesophagus. It is indeed most probable that the an- 
terior, pear-shaped part of the intestine, concerning which 
Rathke (/. c. p. 830-31, t. 11. figs. 16 & 17 a) expresses himself 
with some uncertainty, is of great importance in the act of 
sucking. It still remains to consider a little more closely the 
instruments by means of which the source of liquid is made to 
flow. 
The stem of the mandibles (s, fig. 4c) forms a very large, 
flat cone, with somewhat sinuated outline, slanting forwards 
and inwards, bearing the palpus near its posterior and exterior 
corner, and lying uncovered between the maxilliped and the 
labrum, as far as the place where it penetrates into the mouth- 
‘tube from the side. At this point it is immoveably adnate, 
and affords thus a firm lateral support for the mouth-tube ; 
but immediately inside it is abruptly contracted and twisted 
half round with a downward bend; and here the membrana- 
-ceous articulation of the stem with the mandibular lobe is 
Ann. & Mag. N. Hist. Ser. 4. Vol. i. 2 
