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 Stoma pod a, we 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 

 of those orthoptera called mantis. In consequence of this 

 conformity, these Stomapods have been called, sea mantes. 

 According to the evidence of M. Risso, 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 Amphtpoda, which is composed of the genus 

 Gammaeus, 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. Risso, 

 " to the argonauts and carinari(B, these Crustacea ma}- be 

 seen when the waters are calm, voyaging along in those living 



