the Mouth in Sucking Crustacea. 17 



It remains to examine the structm-e 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 figm-e 

 of a buttonhole. The two romid, naked, soft, and slightly 

 folded lobes of the tongue, which resemble fiat 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 sufiicient strength to cause a liquid to ascend 

 into the oesophagus. It is indeed most probable that the an- 

 terior, pear-shaped part of the intestine, concerning which 

 Rathke (/. c. p. 30-31, 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. 4 c) 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. d- Mag. K Hist. Scr. 4. Vol.i. 2 



