ON CRUSTACEA. 
315 
movements, vary their direction, going sideways and back- 
wards, especially by means of the leaflets of the extremity of 
their tail, which, forming a fan, appear to be more particu- 
larly destined to strike the water in front, and to carry the 
body backwards. The two scales with which their external 
antennae are accompanied, are also useful to them under 
these circumstances. The sort of rostrum, or advanced and 
denticulated bill, which their front presents, is probably a 
defensive weapon ; but we cannot believe, with Rondelet, that 
it is capable of stopping fish of small bulk, and still less of 
killing them. 
With respect to the order of Stomapoda, vve have very 
little to say. They are all marine, and inhabit, in preference, 
the countries situated between the tropics, and do not ascend 
beyond the temperate zones. M. Latreille, though he has 
seen a great number of individuals, has never met with one 
that carried eggs. Their habits are totally unknown ; only, 
that it is without doubt, that those which are provided with 
claws, make use of them to seize their prey, after the manner 
ot those orthoptera called mantis. In consequence of this 
conformity, these Stomapods have been called, sea mantes. 
According to the evidence of M. Ilisso, they remain at great 
depths, on the sandy and muddy bottoms, and couple in 
spring ; but some other stomapods less favoured as to nata- 
tory appendages, having, moreover, the body very much 
flatted, and much more extended in surface, live habitually 
at the surface of the waters, and move there very slowly. 
In the order Amphipoda, which is composed of the genus 
Gammarus, the Crustacea called Phronima present a re- 
markable peculiarity. They are small animals, which have 
for their domicile the interior of the body of divers soft 
radiata, such as the medusae ; “ Similar,” says M. llisso, 
“ to the argonauts and carinaria , these Crustacea may be 
seen when the waters are calm, voyaging along in those living 
