COMPABAT1VB .\\ATOMY. 



163 



of the back, and called in the aggregate oephalo-thorax, is given 



llutration. in comparing the segmentation of Crustacea 



with iiiHoota, it ia thought that the oephalo-thorax, with it* 



i oyoa, two pairs of antenna, foot-jaws, and walking legs, 



!i<is not only to the head and thorax, but also v. 

 it of the abdomen of insect*. In aooorduuoe with thiH 

 >t ion, the part which succeed H to thin, and whirh in tin- 

 prav. a ia a largo muscular tail, is called the post-abdomen. AH 

 haa been before remarked, the post-abdomen haw limbs, which in 

 not tho caao with insects or spiders. The position of the prin- 

 cipal internal organs would bear oat tho conclusion that tho 

 part corresponding to the front part of tho abdomen of insects 

 lies under this great shield. Under it lie the whole of the 

 stomach, generative organs, large liver, and heart, the main por- 

 tions of which all lie in the limbless after-part of tho insect. 

 Tho mouth opens under tho animal, and it is so covered with the 

 great number of pairs of jaws, with their flaps and fringes which 

 guard it, that it is difficult to disclose it. Tho throat leads up- 

 wards into a capacious oval or cubical stomach, whose walls are 

 stiff and strong. It is lined internally with a pile of stiff hairs 

 directed backward, and has at its hind part, from which the in- 

 testine proceeds straight to the tail, three strong teeth which 

 masticate tho food. This tooth apparatus is worked by muscles 

 which run from tho outer wall of the stomach to the shelly sides 

 of tho body. These muscles are under the control of the animal, 

 and are worked at will. Such an arrangement is very rare, as 

 in most cases the working of tho stomach is quite out of the 

 control of tho creature to which it belongs. 



Tho circulatory system is more definite, compact, and perfect 

 than that found in insects. The heart, instead of being long 

 and divided into chambers, is an oval bag which sends vessels 

 forward to the eye, head, antennae, and stomach, sideways to tho 

 two large lobes of the liver, and downwards through a great 

 trunk which divides into two ; one running to the gills and legs, 

 and the other backward to the tail. The blood from the gills 

 finds its way into spaces lying immediately under the shell, 

 which all communicate with one another, and the largest com- 

 municates with the heart by slits in the sfde of that organ. This 

 higher and better developed blood system is rendered neces- 

 sary by tho breathing organs being confined to certain definite 

 parts the gills. In those Crustacea where there are no gills, 

 the circulatory system is not so perfect. The gills are organs 

 which sprout from above the basal joints of the walking legs. 

 In the lobster there are several to each leg. They consist of a 

 tapering triangular stem upon which a vast number of little 

 tubular projections are set. These are of thin membrane, and 

 are supplied internally with blood from an artery which mounts 

 the stem, diminishing as it ascends, while tho aerated blood is 

 discharged into a vein, which also lies in the stem and enlarges 

 as it descends. Although these organs are essentially gills or 

 outward extensions of the integument, yet they are too delicate 

 to be exposed to the casualties of the outer world. They are 

 therefore included under the dorsal shield. In the lower orders, 

 however, they are exposed and attached to the members of 

 other segments of the body. The gills are thus included in a 

 chamber under tho shield. It is of course essential that a free 

 stream of water should pass over them, and to effect this there 

 are two orifices which form the entrance and exit of the water. 

 The entrance lies forward on the side of tho mouth, and it has 

 covering it a large flap from the second pair of maxillae or foot- 

 jaws, which is continually worked so as to drive the water in- 

 wards. The exit is a long slit behind and below the chamber. 

 Aeration is maintained not only thus bnt by mechanical means 

 within the chamber ; for at the top of each leg there is, besides 

 the gills, a long, stiff, leaf-like projection, which passes up be- 

 tween the gills, and as the animal walks this stiff flabellum 

 squeezes the gills, and so renews both the water without and tho 

 blood within them. The position of these gills and flabellum 

 will bo seen in tho illustration. 



The nervous system is like that of the whole of the homogan- 

 gliata, but the degree in which the chain of double nerve-knots 

 is dissevered and gathered together varies vastly. Sixty pairs 

 of ganglia are found in one species, and in the crab all except 

 tho brain, which is situated above tho mouth, and supplies 

 nerves to the organs of sense, are gathered into one nervous ring. 

 The eyes of these higher Crustacea are on the same plan as 

 those of insects. They are, however, set upon jointed movable 

 stems. A discussion of the claims which certain organs in the 



antenuo of the lobster have to bo considered the nose and ear, 

 will be fonnd in the lesson* concerning those organs of "unit 

 The enormously long antennae of some of these ore* tares which 

 are well supplied with nerves must be efficient organ* of sense 

 in tho denae dark waters of the ocean. 



The preceding remarks only apply to those higher fy'H^ of 

 the Crustacea included under the class Deoapoda. We will now 

 very briefly notice the remaining orders, which contain "."im^u 

 of very different structure. 



Amphipoda. The eyes of these are not stalked, bnt their 

 surface ia on the same level as the rest of the body. The first 

 joint of tho thorax is made fast to the head, but the two follow- 

 ing as well as all tho abdominal segment* are free. Only the 

 first pair of limbs are transformed into foot-jaws. The gills are 

 in the form of membraneous plates, set on the middle thoracic 

 segments. These are never covered in by the shell, but are 

 freely exposed. 



The example of this order, which tho reader will moat likely 

 have fallen in with, is the sand-hopper, which is found in countk-Ha 

 multitudes at tho edge of the sea. The liveliness of these 

 animals is remarkable, as they are constantly turning somer- 

 saults backwards, and in BO doing, jerk themselves violently 

 into the air. 



Isopoda. These are segmented like the foregoing, and their 

 gills are like them, but they are set, not on the middle, but the 

 tail segments of the body. The woodlioe (Oniscidca), which, 

 when touched, roll themselves into a perfect ball, are good ex- 

 amples of this order. 



Pcecilopoda. These have their backs covered with two 

 shields, one behind the other, which extend on each side far 

 beyond the body which lies beneath them. They have also a 

 hard, sharp tail-piece, which is made use of by the savages of 

 the Moluccas to form the spear. 



Branchiopoda. These Crustacea have one to three pairs of 

 jaws. Their thorax is undeveloped, and with a variable num- 

 ber of abdominal segments to which gill-feet are attached. 

 Frequently the body of these is covered by a shell. The cypria, 

 which is found in almost every pool of water, belongs to this 

 order. The shells of these are often beautifully marked, and as 

 they are found preserved in geological strata, they have been 

 very useful in determining the nature of those strata. Thus, 

 wherever a cypris is found, it is considered certain that the earth 

 was deposited from fresh water ; and the nature of their mark- 

 ings is so distinct, that the different species are easily distin- 

 guished by the shell alone. 



Entomostraca. These have a well-developed cephalo- thorax. 

 Their abdominal feet are split into two portions, and they are 

 without distinct organs of respiration. To this order belongs 

 the cyclops thus named on account of its centrally situated 

 single eye. Many also of those shapeless parasites which cling 

 to the gills, eyes, and mouths of fishes, are classed with the 

 Entomostraca. 



Cirripedia. In these, the two sexes are combined in one indi- 

 vidual. They are fixed animals, and have a mantle which is 

 furnished with plates of carbonate of lime, like the bivalve 

 shells of molluscs. These are the barnacles. They were long 

 thought to bo molluscs, but have all the essential features of 

 true Crustacea. 



We have found it convenient to descend in the scale while 

 describing these orders ; but it must not be forgotten that tho 

 order should be reversed in a tabular classification. Thus : 



1. Cirripedia (cirrus, a, tendril; pedes, feet), animals with 

 tendril-like feet. 



2. Entomostraca (trrona, insect ; offrpaKov, shell), shelled in- 

 sects. 



3. Branchiopoda (branchiae, gills; TOUI, foot), gill-footed 

 animals. 



4. Pcecilopoda (irotKi\os, varied; irovs, foot), animals with 

 variously-shaped feet. 



5. Isopoda (iffos, like ; wovj, foot), with like (thoracic) feet. 



6. Amphipoda (a/j.<pt, both; irous, foot), animals with both 

 kinds of feet on the thorax, i.e., with walking feet and gill feet. 



7. Decapoda (otica, ten ; -KOVS, foot), animals with ten walking 

 legs. 



Another order called Trilobites inhabited our globe at that 

 very remote period when the primary strata were deposited ; not 

 one of them have survived that period. Their remains are, 

 however, found in great multitudes, and the number of species 



