TO THE ARTICULATED ANIMALS. 407 



of a membranous piece beneath the labrum, which has the same reference to it as the 

 mentum has to the labium. 



In Hemiptera and Diptera the mandibles and maxillse are represented by scaly pieces, 

 in the form of setae or lancets, received in a tubular elongated sheath, which is either 

 cylindrical and articulated, or elbowed, and terminated by fleshy lip-like pieces. In 

 these insects the mouth becomes a real sucker. In other suctorial insects (Lepidoptera) 

 the maxillae alone are elongated, conjointly forming a tubular and very slender instru- 

 ment like a long tongue, spirally folded up at rest, the other parts of the mouth being 

 but very slightly developed, [except the labial palpi]. Sometimes, as in many 

 Crustacea, the fore-legs approach the maxillae, taking their form and exercising their 

 functions, so that the maxillae may in such cases be spjd to be multiplied, and some- 

 times it may even occur that the real maxillae are so much reduced in size that the 

 maxillary feet or foot jaws (pieds-machoires) entirely replace them. But, whatever 

 may be the modifications of these parts, they may always be recognized, and these 

 variations reduced to a primitive or general type. [This kind of reasoning may appear 

 fanciful to persons who have not studied the comparative anatomy of these lower 

 animals, but there are so many instances in which feet are transformed into jaws, and 

 jaws into feet, that it is impossible not to arrive at the conclusion that these organs 

 are but modifications of each other. For instance, in the crabs there are three pairs 

 of foot-jaws and five pairs of legs, whilst in the jumping shrimps (Amphipoda) there 

 is only one pair of foot-jaws, the number of legs being increased to seven pairs by the 

 addition of the two outer pair of foot-jaws. The genera Sergestes, Sicyonia, and 

 Acetes amongst the Shrimps still more clearly prove this, for here the typical number 

 of legs is five pairs, but the same kind of modifications occur. In the winged insects 

 it is quite sufficient to examine the lower lip of a grasshopper, cockroach, or white ant, 

 to perceive at once that it consists of a pair of small maxillae soldered together, the 

 ligula (or labium, as it is restrictedly called by some authors) consisting of two inner 

 lobes, and two galea;, with two labial palpi : if, therefore, we consider the internal lobe 

 of the maxillae as a palpus, the labium in these insects will possess four palpi and two 

 inner lobes. If we adopt this principle, we must suppose that as each leg-bearing 

 segment is furnished with a pair of limbs, the head is a compound segment, furnished 

 with several pairs of limbs, being the analogues of legs, and such is the view entertained 

 by some of the most celebrated of modern entomologists. The same principle Latreille 

 considers to be equally applicable to the antennae, or at least to the inner pair of these 

 organs in the Crustacea, and hence the Arachnida and Myriapoda are not, in this 

 respect, anomalous exceptions to the principle. 1 



THE FIRST CLASS OF ARTICULATED ANIMALS WITH 

 ARTICULATED LEGS. 



CRUSTACEA. 



The Crustacea are articulated animals, provided with articulated legs, respiring by 

 branchiae (a kind of gills), covered in some species by the sides of the carapax or shell, 

 and external in others ; but which are not inclosed in particular cavities of the body', 

 receiving the air by means of orifices in tiie surface of the skin. Their circulation is 



