244 CRUSTACEA. 



the water, is perfectly pellucid, although it almost in- 

 variably becomes semi-opaque when dry. Among the 

 species, most numerous in individuals, that commence 

 their lively evolutions .towards the decline of day, on the 

 calm bosom of the ocean, may be mentioned Hyalaa 

 longirostra, of Leseur, and the beautiful and delicate 

 Hyalaa trispinosa, of the same naturalist. 



On the 10th of June, 1843, we slowly sailed through 

 the Sunda Straits, the tranquil waters of which were 

 crowded with myriads of diaphanous Crustacea, of the 

 genera Erichthw, Phronima, Stenosoma, Alima, Nerocila, 

 Idotea, Spheroma, and others, creatures ever sparkling 

 beneath the wave, and glittering, as their glassy shields 

 reflect the rays of the sun. They swim leisurely in dense 

 strata near the surface, sinking, however, when the sea is 

 at all ruffled. M. Risso says, " they empty the pellucid 

 discs of certain Acaleplice to serve them as canoes," 

 which curious circumstance I have been several times 

 able to confirm. These small isopodous, horny, and 

 generally-transparent Crustaceans, do not swim like the 

 Crangon, with the belly upwards, and by sudden jerks 

 backwards, but propel themselves steadily onwards by 

 repeated contractions of the post-abdomen, and natatory 

 caudal appendages. They are exceedingly predatory and 

 voracious, occasionally seizing Medusa of greater bulk 

 than themselves, holding them in then- prehensile jaw- 

 feet, and tearing them in pieces with their mandibles. 



On these occasions, one must naturally be impressed 

 with the astonishing fecundity and diversity of form 

 exhibited throughout creation. Each portion of the 

 large masses of floating weed consists, when carefully 



