134 BRITISH PALALOZOIC FOSSILS. [Crusracgra. 
food), from which, as in all the Annulata, a straight intestine leads to the anus; there is a well-developed 
liver. Cireulation, by a well-developed heart placed behind the stomach, of mixed blood, which imbibes the 
chyle from the intestine. Respiration, in the higher groups by gills, which are modifications of the flabelli- 
form appendage of certain legs; in the lowest, apparently by the whole surface of the body, without special 
organs. Nervous system, on the plan mentioned when first speaking of the subkingdom Articulata (p. 127). 
Hearing: the higher groups hear well, the ear being situated in the base of the second pair of antenn:. 
Sight: a few parasitic groups are blind in their perfect state, but nearly all the rest have perfect eyes 
either simple, or what I have termed semicompound. The simple eyes are small, two or three in number, 
of a single set of lenses, each eye covered by a round, smooth cornea; the semicompound eyes are an 
agglomeration of simple eyes, each with its set of lenses and separate twig of optic nerve, the group of 
eyes covered by one simple, smooth, external cornea (true compound eyes, having a separate facet of the 
outer cornea for each eye beneath). Reproduction: sexes always distinct and in different individuals. 
The class Crustacea is naturally divisible into the five following orders, commencing with the lowest 
in organization: Ist, Cirripedia, or Barnacles; 2nd, Suctoria, or the little parasitic Crustacea with tubular 
mouths; 83rd, Lntomostraca; 4th, Edriophthalma; 5th, Podophthalma, or most highly organized, and having 
pedunculated eyes (Crabs, Lobsters, &c.) 
3rd Ord. ENntromosrraca. 
= Aspidostraca, Burm. 
The little crustacea which compose this order are very variable in their characters; they include a 
vast number of the most minute crustacea known; all those enclosed in a bivalve shell-like carapace (an 
extension of one of the cephalic rings) belong to it. They have no gills, but breathe by flat, membraneous, 
vascular vesicles attached to the thoracic extremities, and representing the flabelliform appendage and palpi 
thereof, modified for the purpose, or by the surface of the body only. The eyes, whether simple or com- 
pound, are covered by a smooth cornea. This order, which is of more importance to the geologist than 
all the rest of the class, is divisible into three tribes, named according to the structure of the feet: 
Ist, Phyllopoda; 2nd, Pecilopoda; 3rd, Lophyropoda. 
Ist Tribe. PHYLLOPODA. 
= Branchiopoda, Edw. 
These have the feet extremely numerous, and, as their name implies, all leaf-shaped; they form ex- 
tremely numerous, thin membraneous lobes, subservient to respiration. 
The tribe is divisible into the five following families :— 
1st, Lymnadiade.—In which the carapace covers the entire body in the form of a vertical bivalve 
shell, opening along the ventral margin; head adhering to the carapace; twenty to thirty pairs of 
feet, two eyes, either separated or united in one median mass (Lymnadia, &c.) 
2nd, Apodiade.—Carapace horizontally semioval, not covering the abdominal segments, which are dis- 
tinct ; sixty pairs of feet (Apus). 
3rd, Tvrilobitade—Head and abdomen covered by separate dorsal shields; thorax of naked segments 
generally trilobed by two longitudinal depressions. Eyes two, large, semicompound, or absent. 
4th, Branchipodiade.—No carapace, all the body rings distinct, naked (Branchipus, &c.) 
5th, Daphniade (or Cladocera).—Body enclosed in a vertical oval shell, the anterior portion forming 
a beak-shaped hood for the head, which bears the single semicompound eye in front, and is separated 
below from the rest of the carapace, which is bivalve; only four pair of foliaceous feet ; first pair of 
antennze small, second pair very large, branched and bristled for swimming. 
