THE L0B8TEB AND OTEEB CRUSTACEANS. 89 



resented by Fig. 95, with three pairs of legs which end in 

 bristles. Finally, the larva attaches itself by its antennae to 

 some rock, and now a strange transformation follows. The 

 body and legs (the number of legs having meanwhile in- 

 creased) are inclosed by two sets of valves, so that the ani- 

 mal appears as if bivalved (Fig, 96), and at last the barha- 

 cle-shape is attained. 



We thus see that the lobster is but one of a host of ani- 

 mals, all of which have the soft parts of the body protected 

 hy a crust, which is jointed or segmented, while their ap- 

 pendages are also jointed; they breathe by gills attached to 

 the outside of the body, and possess tiuo pairs of antenncs. 



Ordbbs op Crustacea. 



1. Feet leaf -like Branchiopoda : Brine Shrimp, etc. 



2. Limbs free; some -psLTSiSitic.. Entomostraca : Cyclops,' Fish-lice. 



3. Body fixed, protected by a 



shell of several pieces Oirripedia : Barnacles. 



4. Thoracic feet leaf -like; tho- 



rax covered by a ca,Ta.^aice.PhyUocarida : Nebalia. 



5. No carapace; eyes sessile ... Kiratfecapoda : Pill-bug, Beach-fleas. 



6. Body partly covered with a 



large carapace Thoracostraca : Shrimps, Crabs. 



Class Podostomata.— This group, represented by the king« 

 crab (Limulus) and the trilobites, is in many respects inter- 

 mediate between the Crustacea and Arachnida. Taking 

 Limulus, the only form now existing, as an example, there 

 are no antennae or true jaws, but the mouth is surrounded 

 by six pairs of legs, with spines at the base. There are at- 

 tached to the abdomen six pairs of broad swimming feet, 

 the last five pairs with gills. The brain supplies nerves to 

 the eyes alone, there being a pair of large compound and a 

 pair of small simple eyes. Our common king- or horseshoe- 

 crab is Limulus polyphemus* 



In the extinct order Trilobita the body is divided 

 lengthwise into three lobes, and consists of distinct seg- 

 ments, in some kinds very numerous, while the legs are 

 jointed, bearing gills. (See essay by Walcott on trilobites.) 



S'See essays by Milne-Edwards, Packard, Kingsley, Lankester, etc. 



