March 1, 1898.] 



KNOWLEDGE. 



69 



Ci-ani/on, an ordinary shrimp (sec Fig. c). But besides 

 being lengthened and widened, the joints may be variously 

 sculptured, as in the fifth joint of the next specimen, 

 which represents the uncommon leg of an uncommon 

 amphipod (vci Fig. dt ; or one joint maybe outdrawn at its 

 apex to overlap the next, thus producing various forms of 

 what is known as a chela or claw. In Fig. < the fourth 

 joint is prolonged ; in Fig. / the fifth joint. These are 

 two pecuUar forms among the Amphipoda. The next 

 example shows the quaintly shaped leg of a deep-sea 

 isopod, where the so-called " thumb " is on the sixth joint, 

 though, owing to coalescence, it looks like the fifth. The 



the legs which come next to them, and the same may 

 be said of the third maxillipeds in the Decapoda. But 

 whether the appendage be adapted for eating, grasping, 

 digging, or walking, its form can easily be referred to a 

 simple linear original, and this applies also to the maxillfe 

 and the mandibles, although in them the leg-like or linear 

 pattern has become strangely disguised. 



The typical appendage was spoken of as consisting 

 principally of a stem and two branches. Other appur- 

 tenances of the stem must be left for future notice, but 

 the second or outer branch claims more immediate atten- 

 tion. As we have seen, it may remain entirely undeveloped. 



/. Swimming Foot of Amphipod. Jc. First Antenna, Liljelorgio. I. Tail-Foot, Apseude-i. m. Maxallipeds, 3, 4, Feneius. 

 n. o. p. Slaxillipeds, 2, 3, and following limb, Sorialla. q. Le^ of Lepas. 



following figure shows the same thing in the more familiar 

 leg of the river crayfish. Sometimes the joints are attached 

 to one another, not end to end, but at various angles, as in 

 the leg of a tropical prawn (st- I'ig. /), which has thumb and 

 finger furnished each with a brush of long hairs, in nature 

 as useful as they are beautiful. Of the limbs here shown 

 none have the outer branch developed ; seme have over 

 the first joint an expansion called a side-plate ; some have 

 gills or breathing organs attached to them ; most have 

 some sort of garniture of hairs and spines ; but these 

 details are omitted as foreign to our present purpose. 

 Most of the figures are considerably magnified portraits ; 

 that from the crayfish is much reduced. 



Crayfishes, lobsters, prawns, and shrimps, all belong to 

 the Ik'capoihi marrura, the ten-footed long-tailed tribe. 

 In these the muscular pleon or tail part, through its 

 strong development, possesses a commercial value and 

 cannot escape observation. The crabs, on the other hand, 

 which have no meat to boast of in the flexed and flattened 

 pleon, are often erroneously supposed to be devoid of tails. 

 That they are not open to this reproach is obvious, since they 

 form the ten-footed short-tailed tribe, Iknipoda brachi/iini. 

 But be the tail short or be the tail long, all these stalk- 

 eyed creatures agree in having, after the mandibles, two 

 pairs of maxillie and three pairs of maxillipeds and five 

 pairs of peds, pods, feet or legs. In this respect one of the 

 sessile-eyed groups— the highly curious Cumacea — agrees 

 with them. But the sessile-eyed isopods and amphipods 

 have, instead of three pairs of maxillipeds and five of legs, 

 one pair of maxillipeds and seven pairs of legs. Upon com- 

 parison, then, it becomes perfectly clear that the appendages 

 of the eighth and ninth segments are strictly homologous 

 throughout the Malacostraca. We may call them maxilli- 

 peds or gnathopods or trunk-legs, according to their difi'er- 

 ences of form and function, but they are none the less 

 essentially equivalent structures. In some of the Amphi- 

 poda and Isopoda the maxillipeds are more leg-like than 



At other times it invites observation, as in the shrimp- 

 like Schizopoda, which bear this name of " cleft-legs " 

 because their trunk-limbs display both branches. But 

 really there are very few crustaceans which do not, in one 

 appendage or another, display them both. Throughout 

 the Amphipoda the first three pairs of appendages of the 

 pleon have a very uniform character. They almost in- 

 variably consist of a two-jointed stem and two subequal 

 lash-like branches. The lashes are constituted of a great 

 many small similar joints, each furnished with a couple of 

 long hairs, and they are generally efl'ective swimming 

 organs {sef Fig. j). In these pleopods, or legs of the 

 pleon, one may imagine that one sees a pattern of 

 crustacean appendage more primitive than the leg-like 

 one before suggested. Both pairs of antennae usually end 

 in lashes. The first pair often has two (see Fig. A-). 

 Occasionally, as in the isopod Ajisfwhs, there are two 

 such lashes at the opposite extremity of the animal, in the 

 last pair of tail-feet {.'^et- Fig. /). Kepeatedly in the 

 triple maxillipeds of the Decapoda, while one branch is 

 pediform, the other has a terminal lash (see Fig. m). In 

 the Schizopoda this structure is to be found not only in 

 the two pairs of limbs which are equivalent to the second 

 and third maxillipeds {^ee Figs. «, o, p), but in all the 

 five pairs which follow {see Fig. q), these being succeeded 

 by five pairs of pleopods, each with two lash-like branches. 

 One abnormal case is often quoted, in which the eye-stalk 

 of a crayfish developed into an antenna-like lash. This 

 has recently been matched by an equally abnormal case in 

 which a " trunk-leg" has been developed on the pleon of 

 a crab. 



From a wide-reaching subject enough has perhaps been 

 culled to lead the indastrious beginner into an engaging 

 path of inquiry — the comparative anatomy of Crustacea. It 

 will be strange if he can avoid drawing the conclusion that 

 at least all the Malacostraca are of a common origin. It 

 will be strange, too, if the cirri, or legs of the barnacle, 



