COMMON CRAYFISH. 163 



They are formed by two flaps, right and left, which are homologous with 

 the pleura of the abdominal somites, and like those pleura have an outer and 

 inner lamella and a free ventral edge. The cephalostegite bears a median 

 anterior projection, the rostrum, and to either side of the base of this rostrum 

 an eye is visible, pedunculate as in all Podophthalmata. 



The two first antennae (antennules), each with an outer and inner 

 division, the exopodite and endopodite, project forwards in front of the 

 rostrum, and have to either side the long annulated second antenna 

 (antenna) with a pointed scale or squame, the exopodite, at its base. The 

 appendages in relation with the mouth, the two mandibles, the two pairs of 

 maxillae, and three anterior pairs of thoracic limbs or maxillipeds, can only 

 be clearly identified by dissection. The third pair of maxillipeds, however, 

 is conspicuous and lies between the first and largest pair of ambulatory 

 thoracic limbs. Of these ambulatory thoracic limbs there are five pairs. 

 Unlike the three pairs of maxillipeds they consist of a single stem, the 

 endopodite, the exopodite being lost in all Decapoda. Each limb of the 

 first pair is very large and is often spoken of as ' Chela/ but is perhaps 

 better termed, with Professor Huxley, ' Forceps.' It is chiefly used in 

 prehension not in crawling, the function of the remaining limbs. These 

 limbs are slender and consist each of seven joints, the typical number in 

 the higher Crustacea, and known, counting from base to tip, as coxopodite, 

 basipodite or basis, ischiopodite, meropodite, carpopodite, propodite, and 

 dactylopodite. 



One of the coxopodites bears the genital aperture in both sexes in all 

 Macrura y the group to which Astacus and the Lobster belong, in the 

 Hermit Crabs and the Arthrostraca or Sessile-eyed Crustacea. The 

 oviducal aperture in the Macrura is on the third coxopodite, and in this 

 specimen a black bristle has been inserted into it. In the male, the aper- 

 ture of the vas deferens is on the fifth coxopodite. The membrane 

 connecting the coxopodite and basipodite is the spot at which the limb 

 separates when the animal throws it off in consequence of either fright or 

 injury. The basi- and ischio-podite are united in the forceps, so that the 

 joints of this pair of limbs are reduced to six. The three first pairs of 

 ambulatory limbs are chelate. The chela or claw is produced by the 

 enlargement of the outer angle of the propodite into a process equal in 

 length to the dactylopodite. Foreign objects are seized between this pro- 

 cess and the moveable terminal joint. The chela of the Scorpion is 

 fashioned on the same plan, but the produced angle of the propodite is in 

 this animal the internal angle. The claw in the Squillidae is of a different 

 type. The two terminal joints of the limb are elongate, the propodite is 

 more or less grooved, and the dactylopodite bends backwards and fits into 

 the groove. Note that the space between the bases of the thoracic limbs 

 becomes wider and wider posteriorly, and that the sternum of the last pair 



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