THE MUSEUM 



NATURAL HISTORY. 



ZOOLOGY. 



Class CRUSTACEA. 



FoEJriNG part of tlie great division of the animal king- 

 dom, the Annulosa, and immediately following the 

 Insects, comes the important class of animals called 

 Crustacea. 



The length to which the article "Insects" has neces- 

 sarily extended, must be our apology for the summary 

 manner in which this most interesting class must be 

 treated. 



Crustaoeons animals are true members of the Arll- 

 culata, having, like insects, no internal skeleton, but 

 generally, on the contrary, an external tegnmentary 

 skeleton, composed of flexible rings or joints, and the 

 body provided with articulated or jointed organs of 

 locomotion. 



They differ, however, from insects in having their 

 respiration carried on by means of gills or branchite, 

 and in having a double or complete circulation of blood. 

 Their body is in general covered with a hard external 

 shell or crust [crusla), from which circumstance they 

 derive their name. This shell has for its base a pecu- 

 liar substance, which is found also in the covering of 

 insects, called chttine. It is commonly called the 

 carapax or carapace ; and in many of the species, as 

 the common crab and lobster, is extremely hard, almost 

 stony, being composed of a large portion of carbonate 

 (jf lime with a small quantity of chitine ; whilst in 

 others, as in the Entomostracous species, it is of a 

 horny consistence, the composition partaking largely of 

 chitine and albumen, and slightly of carbonate of lime. 

 Tliis carapace being divided into rings or joints, the 

 animals, even in those which have it hardest, possess 

 a considerable degree of freedom of motion. The nor- 

 mal number of these rings or joints is twenty-one, but 

 two or three are often blended together or soldered into 

 one. To each of these joints except the last, there is 

 attached a pair of members, the forms and uses of which 

 vary much in the different species, according to their 

 age, &c. These members are always divided into joints 

 or articulations, the number of which is very various, 

 and are covered with the same envelop or crust as the 

 body itself. Crustacea possess the power of casting off 



or shedding their carapace at certain periods. During 

 the time the animal is growing in size, their external 

 covering does not increase in equal proportion. In 

 order, therefore, to permit the growth of the body to go 

 on, this solid envelop is thrown off, and a new one ex- 

 actly fitted to the increased size of the animal is formed. 

 When young these periodic moultings, as they are called, 

 are of frequent occurrence, and it is wonderful to see 

 the perfect manner in which the covering of the body, 

 feet, antennae, &c., is thrown off, without any portion 

 of it being destroyed or broken. 



Crustacea respire by means of organs analogous to 

 the gills of fishes, and called, as in them, branclda;. 

 Tliese organs are either exclusively formed as gills, or 

 they are members modified and adapted for that pur- 

 pose. By their means the animals separate the oxygen 

 contained in solution in the water in which they live. 

 They vary much in number and in shape in the differ- 

 ent species, but are generally placed near the base of 

 the legs, and are protected by the border of the cara- 

 pace, though a few have them placed externally. 

 The blood in the Crustacea is colourless, and when 

 observed by the microscope is seen to contain a gi'cat 

 quantity of albuminous globules suspended in it. There 

 are no absorbent vessels, and it appears that the chyle 

 is carried into the blood by imbibition. Crustacea 

 possess a heart, which by its muscular action propels 

 tlie blood over every part of the body, through a well- 

 developed system of arterial vessels. After it has 

 served for the nourishment of the different organs, it is 

 carried into the veins, which are rather reservoirs or 

 lacuncE than vessels properly so called, and from them 

 through the branchia; or gills. After traversing these 

 organs it returns directly to the heart, to run anew the 

 circle just mentioned. The nervous system of the Crus- 

 tacea consists of a chain of ganglions, more or less 

 numerous, united by nervous cords. The normal num- 

 ber of these ganglions is equal to that of the joints of 

 the body; but sometimes they are double, and at others 

 they appear to be much fewer in number, owing to 

 several being united together into one mass. From 



