404 CHARACTERS OF INVERTEBRATE ANIMALS 



this gill-cover is cut away the gills will be seen, and it can 

 readily be shown that they are of three kinds, attached to 

 different regions of the body. It need only be mentioned here 

 that some of them are limb-gills, so called because they are borne 

 by the bases of certain limbs, among which are the three first 

 pairs of legs. When one of these is carefully detached it will 

 be seen that a thin plate (gill-plate) projects from its base, and 

 that to that plate a feathery gill is attached. The fourth thoracic 

 segment bears the most conspicuous appendages of all, i.e. the 

 great pincers, which are constructed on the same lines as the 

 nipper-bearing walking-legs, and like them are provided with 

 gill-plates and limb-gills. 



The three first segments of the thorax bear appendages 

 modified for chewing, and called foot-jaws, the name suggesting 

 the idea that they were once locomotor in function but have 

 acquired new duties. The last or third pair of foot-jaws are 

 decidedly leg-like, but are provided with stiff bristles where they 

 bite against one another. An outer branch is present, though 

 small, and gill-plate with limb-gill will be readily recognized. 

 The middle or second foot-jaws are like the preceding, but a 

 good deal smaller, while the first foot-jaws are very delicate, 

 with broad biting basal stalk, small outer and inner branches, 

 and gill-plate devoid of gill. 



Head Appendages. — These consist of three pairs of jaws 

 behind and two pairs of feelers in front. The jaws, beginning 

 with the last pair, are named, as in an insect, second maxillae, 

 first maxillae, and mandibles. The second maxillce somewhat 

 resemble the first foot-jaws, but gill -plate and gill are absent, 

 while the outer branch is broadened into an oval "baler", which 

 lies in the front of the gill-chamber, and by its constant scooping 

 movement brings about a forward movement of water over the 

 gills. T\\Q first maxillcs are still more delicate, and are reduced 

 to a two-jointed basal stem and an insignificant inner branch. 



The same parts are present in the first jaws or mandibles, 

 but their proportions and texture are very different. The basal 

 joint of the stalk is broadened into a hard biting-piece, strongly 

 toothed on its inner margin, while the second joint of the stalk 

 together with the inner branch make up a small three -jointed 

 " palp " which probably has sensory functions. 



Owing to an upward bend of the region in front of the 



