72 THE YOUNG OF THE CRAYFISHES ASTACUS AND CAMBARUS 



While the absence of the locomotor sixth pleopods in the first and second 

 stages of crayfish young might be regarded as indicating that these stages had 

 never been represented by free ancestral forms but were merely embryos prema- 

 turely removed from the egg to live a pseudo-larval life upon the mother's ple- 

 opods, yet the near marine relative of the crayfish, the lobster, likewise has no 

 available sixth pleopods in its first and second stages though free and even pel- 

 agic, and we are free to assume that the first stages of the crayfish are modified 

 representatives of free ancestral larvae. 



Such ancestors may have been much like the lobster and at present the 

 third larval stage of the lobster is like the third in the crayfish in that the 

 sixth pleopods are then first expanded; but it is not till the fourth larval stage 

 in the lobster that the length of antennae and perfection of tail-fan is developed 

 enough to be comparable to the third stage in the crayfish. While the first and 

 second stages of the crayfish are well adjusted to life upon the mother's ple- 

 opods, the first, second, and third stages of the lobster are well fitted for pelagic 

 life in retaining the locomotor exopodite of the pereiopods and short antennae. 

 The fourth stage of the lobster first acquires long antennae, perfect tail fan and 

 reduced exopodites, and is in the main very like the third stage of the crayfish ; 

 in fact the adult state is rather suddenly imitated in the third stage of the cray- 

 fish and in the fourth stage of the lobster. If these two stages are supposed 

 to be homologous we may suppose that the process of reduction of metamor- 

 phoses that has already gone so far in the lobster (Herrick, '95), has advanced 

 a little more in the crayfish so that only two larval stages exist where there used 

 to be three. 



The greater length of the antennae in the earlier stages of the crayfish and 

 the earlier presence of the 2nd-5th pleopods may be imagined as secondary 

 changes from the lobster-like state, possibly connected with development of 

 crawling habits and abandonment of pelagic life, for the long antennae would be 

 of more use to a creeping animal and the pleopods perhaps of more use in con- 

 nection with respiration and sensory examination of water to an animal liv- 

 ing on the bottom, or possibly in holes, and needing a supply of water to the gill 

 region and information of the nature of the material beneath the abdomen. 



In the departure of crayfish from marine lobster-like ancestors the changes 

 have been in the abolition of most of the free swimming contrivances, the ac- 

 quisition of some arrangements better fitted for a crawling life and finally the 

 retention of embryonic states along with special new adjustments in connec- 

 tion with a life of quasi-parasitism upon the mother. 



We imagine the ancestral crayfish to have given up pelagic larvae, to have 

 developed crawling larvae, and with these to have lately begun an association 

 of larva and parent that forms an early phase in family life. If continued in 

 the same lines there might ultimately result a crowding back of hatching nearer 



