190 



NATURAL HISTORY. 



that all these Arthropod animals have one pair of jointed limbs to each ring or segment of their 

 bodies; consequently, if there be one ring, whether of extra size or no, having more than one pair of 

 limbs attached to it, one is justified in at once concluding that this particular joint or plate is 

 composed of several separate rings soldered together. And this conclusion is really borne out by 

 study of the general structure of the clas^, and also by examining the larvae, which, like those o'.' 

 insects, often show more clearly the true number of rings composing the body 

 because they are not soldered together so compactly as in the adult animal. 



Take a Lobster, and try to comprehend its armour-plated body in detail. 

 It appears to be made up of two chief divisions, namely, a large head-shield 

 (ca), having a strong rostrum (r) or prow in front, like a ship's bowsprit, 

 and behind this seven distinct and movable rings or segments, the seventh and 

 last united to the sixth, and marked tin. the engraving (Fig. 1). Each of these 

 body-rings, save the hindmost, which is called the telson, has one pair of ap- 

 pendages attached to it, those of the last but one being expanded to form 

 a broad and powerful swimming tail. The headflhield, however, covers no 

 fewer than fourteen pairs of appendages, so that there ought to be as many 

 as fourteen rings in this division, all more or less soldered together, thus 

 making, with the body, no fewer than twenty -one segments. 



In fact, this is the number of body-rings most generally found among tho 

 various members of this class, and although instances occur among the extinct, 

 group of the Trilobitcs and the living forms of the order Brcmchwpoda, (sec* 

 table, p. 196), in which a greater number than twenty-one segments can bo 

 detected, and also among the higher Decapoda (Crabs and Lobsters), in which 

 fewer than twenty-one segments are visible, yet the former must be treated as 

 " exceptions which prove the rule," whilst in the latter it can be shown thai, 

 in their young or larval stages they do possess the full number of twenty-one* 

 segments, but that in the adult animal some of these become permanently 

 soldered together. It is found convenient to treat the first seven of these 

 rings as forming the head (cephale), and the second seven as the thorax, or 

 middle body, and the last seven as the abdomen. But in the Crab and 

 Lobster the seven body-rings forming the thorax are entirely concealed 

 beneath the great overarching carapace or head-shield, which is really com 

 Fig. 1. DIAGRAM OF THE p Ose( j o f the rings of the front segments of the head enormously developed, so 



1JODY SEGMENTS OF A A ' . ,11111 



LOHSTER. as to cover over all the others in the Crab, and all but the last seven 



ca, the carapace, covering both abdominal rings in the Lobster. If, however, this roof-like head-shield bo 



the seven cephaDc and th- 1,1,1- 11 c ^i .1 t_ j 



seven thoracic segments; r, carefully removed, the thin walls ot the seven thoracic body-rings aro 



the Rostrum ; 16, rings of * 



and fast "in", : ni'so'cafi^d''"^ actually to be seen there, only they are concealed beneath the overarching 

 !'iat<To'n Tadf'side"^ The head-shield each rin ivin attachment to one out of the 



tail are the modified append- 



seven pairs 





head-shield, 



e - ..,-,, , -, . .,.,... / ^i i> i i 



iieiimging to the sixth ot jointed legs belonging to this division ot the animal s body. 



The first and most anterior pair of appendages in a Crustacean is com 

 posed of the two eye-stalks, each bearing an eye (Fig. 2, A) at its extremity. The mouth, which is 

 placed in the centre of the head, just beneath the rostrum, has a small median plate in front,, 

 called the labrum (Fig. 2, B), or upper lip-plate, and a two-lobed piece behind it, called the meta 

 stouia, or lower lip (Fig. 2, c). 



The second pair of appendages, following in order after the eyes, are called the antennule?, 

 or inner antennae (Fig. 2, D), each consisting of a protopodite (or root-footlet) bearing two long, 

 slender, many-jointed feelers, representing the two parts of an ordinary jointed limb, one being called 

 the exopodite (or outer footlet), and the other the endopodite (or inner footlet). These undoubtedly 

 serve as important organs of touch, and at the base of each in the protopodite is a small sac, opening 

 externally by a narrow cleft guarded by hairs. At the bottom of this sac is a prominence wherein 

 the auditory nerve terminates, and on which are very delicate hairs with silicious particles which have 

 (apparently) found their way in from the exterior. The third pair of organs are the two great feelers 

 or outer antennae (Fig. 2, E), which exceed in length the entire body of the Lobster. Like the 



