188 CRUSTACEA. SIPHONOSTOMA. 



posteriorly, with the lateral borders armed with moveable and alternate spines; 

 and below are five pairs of leaflets or broad swimming-feet, provided with branching 

 on the upper surface. 



Gen. LIMULUS, Muller. Monoculus, Lin. 

 Shell horny, thin and hollow, the anterior portion formed of a 

 shield with three carinse, deeply notched behind ; the second 

 trapezoidal, with moveable spines and teeth on the lateral 

 margins, and notched behind ; tail long, slender, ensiform, 

 and terminated by two long filaments ; two compound eyes ; 

 mouth below the posterior margin of the head, longitudinal, 

 and narrowed by appendages ; feet numerous, foliaceous, the 

 two anterior largest. 



L. Polyphemus, Lat. Three spines on the ridge of the shell ; tail 

 triangular,, dentated below ; colour yellowish white in young in- 

 dividuals, brownish -black in the old. Inhabits American coasts. 

 Desrn. Crust. 354. 



ORDER IX. SIPHONOSTOMA. 



With a syphon or proboscis, but sometimes concealed, and hav- 

 ing a papilla for suction ; feet beyond six or seven pairs ; 

 shell of one piece, soft, membranous, and not covering the 

 body entirely. 



The Crustacea of this order are parasitical, and have two eyes, but indistinctly vi- 

 sible in some. The sucker or syphon is formed by the labrum, the labium, and 

 perhaps some other part. 



FAMILY I. CALIGIDES. 

 Shell distinct, and body more or less oval. 

 Gen. 1. ARGULUS, Muller, Lat. Monoculus, Lin. 

 Shell almost membranous, semitransparent, depressed, general- 

 ly oval, and slightly notched on each side anteriorly ; two 

 apparent compound eyes ; four antennae, the upper ones with 

 a hook at their base ; abdomen cylindrical, with two small 

 corneous hooks anteriorly ; tail formed by a horizontal lamina, 

 terminated by two rounded lobes; feet twelve, of various forms, 

 the anterior pair largest and terminated by a circular disc. 



A.JbliaceiiSj Jurine. (M. foliaceus, Lin.) Body oval, depressed, 

 greenish yellow, and semitransparent. 2J lines long. Inhabits 

 stagnant waters in Europe Desm. Crust. 352. 



Gen. 2. CALIGUS, Muller. 



Body depressed, covered with a membranous shell ; abdomen 

 narrow, of an elongated oval form, or almost square, and ter- 

 minated by two elongated and cylindrical setae ; eyes two, at 

 the internal base of the antennae ; an obtuse beak below the 



