COMMON CRA YFISH. 



171 



The second appendage has a protopodite divisible into a coxopodite and long 

 basipodite. The endopodite consists of a large basal piece and terminal jointed 

 filament. The apex of the basal piece is prolonged upwards as a plate to the inner 

 side of the filament, and the inner edge of the plate is rolled upon itself. The 

 exopodite is present and has the usual structure. Both pairs of appendages are 

 used for the transmission of sperm : see p. 186. 



Both the male and female alike possess no appendages to the first abdominal 

 segment in the Parastaridae the Crayfishes of the S. Hemisphere. 



The telson is a plate moveably articulated to the last somite of the abdomen. 

 It is divided in nearly all the Potamobiidae or Crayfishes of the N. Hemisphere, but 

 not in the Parastacidae, by a transverse suture, so that the posterior half is moveable 

 upon the anterior half. The anus is situated on the ventral surface of the basal 

 portion. There is some question as to what the telson represents. Hartog (British 

 Assoc. Reports, 1882, p. 575) believes that it represents the last somite of the 

 Nauplius a post-anal plate united to the furcae anales, the latter representing 

 paired terminal outgrowths elsewhere developed into limbs by the formation of 

 joints. He points out that in the Copepoda in his view the primitive Crustacean 

 group the anus is a terminal dorsal slit; that the tergum of the last somite 

 forms a supra-anal plate, whilst the furcae project one on either side of the anus. 

 Supposing the supra-anal plate to become adnate to the furcae, the anus becomes 

 first terminal and ventral, and finally by growth of the plate ventral. He homolo- 

 gises two setose knobs projecting at the sides of the telson in Astacus with the 

 furcae. It has been pointed out by Claus (Untersuchungen, &c. supra, p. 12) that 

 in the Protozoaea- < s>\&<gt of Penaeus the anus is terminal between two furcal processes. 

 In its youngest Zoaea a short transverse bridge (= supra-anal plate of CopepodaT) 

 connects the furcae dorsally above the anus. During subsequent growth this bridge 

 enlarges, the anus becomes more and more ventral, and the furcae are lost, be- 

 coming the two posterior setigerous processes of a broad bilobed terminal plate. 

 In a young Phyllosoma (cf. Claus, op. cit. p. 51) the same facts may be observed, 

 but the processes become obsolete, the setae alone persisting at the outer angles. 

 There is much variety in the shape of the last somite of the abdomen in 

 Crustacean larval forms. It is very frequently a broad bilobed plate, more rarely, as 

 in the young Astacus itself, a simple plate. In both cases the anus is ventral. 

 Claus concludes that the telson represents the terminal furcal somite of the abdo- 

 men in Phyllopoda. It is possible that it may represent a region rather than a 

 somite. For in Nebalia (Claus, Z. W. Z. xxii. 1872, p. 329) there are two somites 

 behind the sixth abdominal somite, the last bearing a pair of furcae and the anus, 

 and in most Phyllopoda the abdomen contains a large number of somites. 



When Astacus quits the egg it only differs from the adult in certain points 

 summarised here from Professor Huxley's account. The cephalothorax is relatively 

 large and convex in shape; the short rostrum is bent down between the eyes; 

 the thoracic sterna are relatively wide ; the chelae of the forceps are slender, and 

 the tips of all the chelae are strongly incurved, the young Crayfish attaching itself 

 by those of the forceps to the empty egg-case : the dactylopodites of the two last 

 pairs of thoracic limbs are hooklike : the first pair of abdominal appendages is un- 

 developed : the sixth is included within the telson, which is a simple broad oval 

 plate usually notched in the middle of its hinder margin. Setae are few in number 

 and are mostly uncalcified prolongations of the cuticle not sunk in pits, and devoid 



