162 



THE POPULAE EDUCATOE. 



tenant the atmosphere. Many are constantly on the wing, and 

 they are known as the class of animals that give vivacity to 

 the still summer air. It maybe further stated that while some 

 larvae of insects inhabit, and some mature forms are found living 

 (though not breathing) in water, all are excluded from the salt 

 water of the ocean. If, therefore, there were no other repre- 

 eentatives of the higher forms of articulates than the classes 

 Myriapoda, Insecta, and Arachnida, this great sub-kingdom would 

 be unrepresented in the vast domain of the salt water. The 

 shores, sand, and mud of the sea-bottom might be the home of 

 inert and crawling worms ; but its limpid mobile waves would 

 be given up exclusively to fish and sea-snails, star-fish, and sea- 

 jellies. It has pleased the great God, who, we have reason to 

 believe, delights in the variety of forms of animal life, that this 

 shall not be the case. Hence we have in the Crustacea a very 

 large class of articulates whose natural home is the salt water. 

 We must, however, here guard ourselves from misleading by 

 stating that marine existence is characteristic of the Crustacea 

 as a class. Many individuals and whole families of them live 

 ashore. Nevertheless, just as insects are made for the air, so 

 are these creatures created for aquatic life. Their manner of 

 breathing is always after the aquatic type. Lest this last sen- 

 tence should not be understood, we must recur again to what 

 has been said about the function of breathing a function which 

 is necessary to almost every animal. This function consists in 

 gaining oxygen or vital air, as it has been well called, for the 

 uses of the system, and in giving up carbonic anhydride, which 

 is the final product of the combination of the oxygen with the 

 constantly changing materials of which the body is composed. 

 Now since oxygen is one-fifth of the air, this gas is readily ob- 

 tained from the air. We have but to expose a delicate moist 

 membrane to air, and it absorbs the oxygen at once. Since a 

 delicate membrane is best protected by being covered, it is suf- 

 ficient that an animal have an internal bag to which the air is 

 admitted and sometimes changed. Owing to the readiness with 

 which gases diffuse themselves through one another, carbonic 

 anhydride, or the spent product, readily passes off while the 

 oxygen gains admission. In water, however, it is different. 

 The thing required namely, oxygen is just the same ; but 

 this is only slightly soluble in water, hence water has always 

 but a very small proportion of this vital element ; moreover, the 

 water offers more resistance to the passing off of the spent gas. 

 Hence it follows that much more water must be constantly ap- 

 plied to the breathing organ to get the requisite amount of 

 oxygen, and the carbonic anhydride must be carried away by a 

 current, and not left to diffuse itself. If a water-animal breathed 

 by internal bags, these bags must be emptied completely by the 

 most rapid and violent contractions, and even then provision 

 would have to be made to keep the orifice clear of effete pro- 

 ducts by passing a current over it. Such an animal would be 

 in the condition of a vessel which has sprung a leak, all of 

 whoso crew must work at the pumps for dear life. As a matter 

 of fact, the economy of nature does not admit of such a state of 

 things, and so all water-animals either breathe through the 

 whole skin or parts of the surface are protruded into the water 

 around, and thus the liquids of the body become aerated. Such 

 protrusions of delicate membrane, containing a circulatory fluid, 

 are called gills ; and our crustaceans are distinguished from in- 

 sects by breathing by gills, when they have any breathing 

 organs at all, instead of by a trachea! system. The Crustacea 

 form a class which seem to run parallel with the insects. The 

 highest of them may dispute with these last the palm as to com- 

 plication of structure, while the lowest stretch downward 'almost 

 to the Protozoa. If, as we have suggested in a former lesson 

 (Vol. II., page 311), the crustacean type is to be traced up from 

 the Protozoa, not through the Annuloida and the Annelida, but 

 directly through the Kotatoria, it is certainly singular that they 

 should have so many points of similarity to insects ; as, for in- 

 stance, in the nature of their integument, their definite joints, 

 and the character of their limbs, jaws, eyes, antenna?, and all 

 the organs of sense. Some may think that insects and crus- 

 taceans have arrived at a similar conformation by different 

 routes. These would say, Given that you have an articulate 

 animal well developed for locomotion, it must assume some such 

 form as is common to the insect and lobster. This statement, 

 however, scarcely carries conviction to the mind. 



We will now give a description of one of the higher and 

 more typical crustaceans, such as a lobster, crab, or prawn, and 



then point out some of the modifications of this type. These 

 creatures are in common speech called shell-fish, but this name 

 is a bad one ; for they are not fish, nor can their investment be 

 called a shell, the word shell being more properly applied to the 

 external secretions of the Mollusca. There is, however, one point 

 of similarity in these two kinds of shell namely, that the earthy 

 salt, carbonate of lime, forms the hard deposit in both cases. 

 This salt, which is the same as chalk, limestone, and Iceland 

 spar, is the one which seems to be the most easily employed by 

 invertebrates to strengthen those tissues which it is necessary 

 should be rendered hard and inflexible for the purposes of the 

 animal. This chalky induration is no doubt taken into the 

 system while dissolved, and circulates with othei constituenta 

 of the nutritive fluids, and only assumes its hard, stony, and 

 solid form when it has reached the integument of the crustacean, 

 and is there laid up in its cells. When once laid down, how- 

 ever, it seems to be but little liable to be re-dissolved and car- 

 ried away again, as the organic substances of the tissues are. 

 Probably the lime is absorbed while in some form more soluble 

 than the carbonate, and only becomes the less soluble carbonate 

 by becoming combined with the carbonic anhydride (C0 2 ), which, 

 as we have seen, is a continual waste product of the system. 

 The reason for this supposition is derived from the fact that 

 when the animal is growing within its stiff hard shell, instead 

 of the earthy particles being partially dissolved and re-arranged 

 with the deposit of more of such particles in the interstices, 

 the whole shell is cast off with the layer of skin which encloses 

 it, and the whole has to be reproduced from below. The whole 

 integument or outer coating of the crustacean is the same as 

 that of the insect. There is the constantly vital inner layer 

 permeated by the blood. Over this is a layer of pigment which 

 gives the colouring to the animal, and which often exudes 

 colouring matter which penetrates the whole of the shell which 

 lies above it ; and lastly, there is the external epidermal layer in 

 which the shell is deposited. This last, with its involved chalk 

 substance, is often of great thickness, and the chalk is laid down 

 in closely applied rods or columns, which lie perpendicular to 

 the exterior of the animal. This epidermis corresponds to our 

 scarf-skin. In its physical character it partakes of the nature 

 of this and of horn. Chemically it is different from either of 

 these, as it burns quietly to a white ash, without either melting 

 or swelling up as horn does. Moreover, caustic potash (KHO) 

 will not melt it, nor will nitric acid (HN0 3 ) colour it yellow, as 

 our epidermis is dissolved and coloured yellow by these several 

 substances. This outer epidermis of the higher Crustacea, which 

 is so thick and hard in some parts, also passes as a thin film 

 over other parts, clothing the whole body and even extending 

 inwards into the alimentary canal. So that when the creature 

 casts off its old coat to allow it to grow, the old slough presents 

 the perfect shape of the living animal, and the coat of the 

 stomach with its internal teeth are also found in connection with 

 the rest. Thus truth is stranger than the wildest fiction. 

 Baron Munchausen, when he could deal in no other way with an 

 enraged wolf, thrust his hand down his throat, and turned him 

 inside out like an old glove ; but even Baron Munchausen would 

 have been taken aback if after having done so he found the 

 wolf none the worse. This exuviation, as it is called, is effected 

 in the larger Crustacea by means of a transverse split which 

 occurs between the great dorsal shield and the succeeding 

 parts of the body. Through this the creature escapes, with- 

 drawing all its limbs from their cases, and when they arc so 

 large and thick at their terminal joints (as the pincers of the 

 crab are) as to prevent their extrication through the proximal 

 joint-casings, these last split up, and so complete denudation is 

 effected. The fresh skin formed below is at first quite soft and 

 flexible, and the fierce truculent crab, with its formidable toothed 

 and strong pincers, becomes for the time a poor defenceless 

 coward, compelled to skulk about among the stones and chinks 

 of the rocks to avoid enemies which a few days before and a few 

 days after it would defy. If the reader will refer to the second 

 lesson of this series, he will find a detailed description of the 

 divisions of the lobster's body and the limbs which are attached 

 to them. It is needless to repeat this description. There are a 

 vast number of species of Crustacea, and the variations in the 

 number of segments, and the number, position, and shape of the 

 limbs, is almost endless. Another species, the common prawn, 

 with the disarticulated appendages of the first fourteen segments 

 of the body, or those which are included under the great shield 



