310 THE CRAYFISH 



open by gonaducts (Ovd} on the dorsal surfaces of the disc. 

 The sexes are lodged in distinct individuals. 



Both eggs and sperms are shed into the water and after 

 impregnation the oosperm becomes a gastrula which is con- 

 verted into a peculiar free-swimming larva ; this undergoes 

 metamorphosis and is converted into the adult form. 



THE CRAYFISH. 1 



In a crayfish or lobster the body is bilaterally symmetrical 

 and is distinctly segmented, consisting of a prostomium and 

 of nineteen metameres. The anterior twelve metameres are 

 united with one another and with the prostomium to form an 

 unjointed portion of the body, the cephalo-thorax (Fig. 78, 

 A, C. T/i.) : the seven posterior segments are free and con- 

 stitute the abdomen (Abd. Seg. i, Abd. Seg. 7). It is very 

 generally characteristic of Arthropods to have the metameres 

 limited and constant in number, and for more or fewer 



/ 



of them to undergo concrescence. 



Another distinctive arthropod character illustrated by 

 the Crayfish is the possession of lateral appendages of the 

 body. These are given off from the ventral region, two pairs 

 being borne by the prostomium and one by each of the 

 metameres, except the last. Moreover the appendages 

 themselves are segmented, being divided into freely arti- 

 culated limb-segments or podomeres. 



1 For detailed descriptions of the Crayfish see Huxley, The Crayfish 

 (London, 1880) : Huxley and Martin, Elementary Biology, new ed. 

 (London, 1888), p. 173 : Rolleston and Jackson, Forms of Animal 

 Life (Oxford, 1888), pp. 162 and 307 : Marshall and Hurst, Practical 

 Zoology (London, 1888), p. 125 : and Parker, The Skeleton of the New 

 Zealand Crayfishes (Wellington, N.Z., 1889). 



