408 
ARTICULATED ANIMALS. 
double, and analogous to that of the Mollusca. The blood is transmitted from the 
heart, situated near the back, to the different parts of the body, where it is conveyed 
to the branchiae, and thence back to the heart. These branchiae are situated either at 
the base of the legs or upon the legs themselves, or upon the subabdominal ap- 
pendages, forming either pyramidal masses, composed of layers of fine plates or clothed 
with setae, or consisting of simple plates in tufts, even in some appearing to consist 
only of hairs. 
The nervous system of the Crustacea (especially investigated by Cuvier, Audouin, 
and Milne Edwards), exhibits two very different appearances, constituting the two 
extremes of the modifications it presents in this class. Sometimes, as in the leaping 
shrimps {Talitrus)^ it is composed of two nervous chords, with knots or ganglions 
at equal distances along the whole length of the body, and sometimes, as in the Crab 
{Maia Squinado), it consists of only two nervous masses, of unequal size, one placed in 
the head and the other in the thorax. Other Crustacea {Cymothoa^ Phyllosoma, 
Palinurus, Palemon, and Astacus), exhibit intermediate formations, showing the 
gradual modifications.* 
The Crustacea are destitute of wings, provided with two face-tted eyes, but rarely 
with simple eyes, and generally with four antennae. They have in general (the 
Poecilopoda excepted) three pairs of maxillae (the upper pair or true mandibles included), 
the same number of foot-jaws, the outer pairs of which become, in many species, real 
feet ; and ten legs, all of which are terminated by a single hook. When the two 
outer pairs of foot-jaws perform the office of feet, the number of legs is [increased to] 
fourteen. The mouth consists, as in insects, of an upper lip, a tongue, but no true 
lower lip comparable with that of insects, the external pair of foot-jaws [the third 
pair, or, where the two outer pairs become legs, the first pair] closing the mouth and 
acting instead of a lip, [thus proving what has been suggested above relative to the 
nature of the labium in insects] . 
Their envelope is generally solid, and more or less calcareous. They change their 
coats several times, generally retaining their primitive formf and their natural activity. 
They are in general carnivorous, aquatic, and their hfe extends through several years. 
They do not become adults until after a series of moultings. With the exception of a 
small number in which these moultings somewhat modify the primitive form, and 
augment the number of locomotive organs, these animals are at their birth (size 
excepted) such as they will remain throughout their life. 
The situation and the form of the branchise, the manner in which the head is 
articulated with the trunk or thorax, the moveable or fixed structure of the eyesf, the 
organs of mastication, and the tegumentary system, form the bases of our distribution, 
and give rise to the following orders in the class, and which are confirmed by the 
observations hitherto made upon the nervous system. 
* [The modifications in the structure of the nervous system of the 
larva, pupa, and imag-o of the same insect, fully confirm this, that of 
the larva resembling that of the Talitrus, whilst that of the imago is 
more analogous to that of the Crab. If we regard the larva as in a 
state of immaturity or imperfection, we should be led to consider the 
Crab as far higher in the chain of nature than the Talitrus, and such 
is the station generally assigned to it, without reference to its nervous 
system.] 
t [This statement has been opposed by Dr. J. V. Thompson, in his 
Zoological Researches and other more recent articles, this writer 
asserting that the Crustacea undergo a series of transformations as 
striking as those of the true insects ; the anomalous animals long 
known under the generic name of Zoea, and which have long perplexed 
Crustaceologists (for want of a perfect investigation of their struc- 
ture), being affirmed by him to be the young of the Crabs and other 
Decapoda. In some cases, however, where- a minute analysis of the 
eggs of different species has been made, a contrary result has been 
obtained, Rathke having dissected the eggs and watched the gradual 
developement of the embryo of the crayfish, and I having dissected 
the eggs of the land crab of the West Indies, the young in both in- 
stances (and in others subsequently observed by Rathke) resembling 
the parents in general appearance.] - . 
t Whence Lamarck divided the Crustacea into the Pediocles (or eyes 
on footstalks) and Sessiliocles (or sessile eyes) . Leach changed these 
names (applying them only to the Malacostraca) into Podopthalma and 
Edriopthalma. Gronovius first employed this character. 
