2l8 



THE 



CRAYFISH. PHYLUM ARTHROPODA 



summer. They do not differ greatly from the adult but have 

 curved tips to the pincers, by which they chng for a time to the 

 empty shell or the abdominal Umbs of the mother (Fig. 150), 

 and are thus protected from enemies and kept from being swept 

 away by currents and so eventually reaching the sea, where they 

 would perish. 



Fig 150.— a, Two recently hatched crayfish holding on to one of the swimmerets 

 of the mother ; B, pincers of the young more highly magnified.— From Huxley. 



e.c. Ruptured egg cases ; en., endopodite ; ex., exopodite ; pr., protopodite. 



ARTHROPODA 



The phylum Arthropoda, to which the crayfish belongs, is 

 not easy to characterise. Its members resemble the Annelida 

 in being bilaterally symmetrical triploblastic segmented Metazoa, 

 in the general plan of the nervous system, and in the possession 

 of coelomoducts. They differ in the great reduction of the coelom 

 and parallel development of a perivisceral hsemocoele, in the 

 absence of nephridia, and in the absence (except for Peripatus) 

 of cilia. As positive features they possess paired jointed limbs. 



