THE YOUNG OF THE CRAYFISHES ASTACUS AND CAMBARUS 71 



bending down of the rostrum between the eyes and the small number and insig- 

 nificance of the telson spines. The imperfect state of the sense organs ; namely 

 the short eye stalks and incomplete eyes, the open unguarded ear-pit, the com- 

 plete or nearly complete absence of sense set;c on the first antenna and the naked 

 character of the other appendages and whole body over which only a few simple 

 spines take the place of the innumerable sensory and locomotor setae of active 

 larval stages. The lack of development and of use of the first and sixth pairs 

 of abdominal appendages ; the imperfect development and shortness of the first 

 and second antenna;; the lack of teeth on the edge of the mandible and the ap- 

 parent lack of use of any of the numerous mouth parts and appendages con- 

 cerned in later stages in getting and eating food. The passive habit as a 

 fixed dependant with physiology like that in the embryo, except for the addi- 

 tion of active respiratory movements; the pose of the body with the limbs and 

 abdomen beneath the thorax and antennae depressed. 



The special characters of the first larva that are neither embryonic nor 

 such as a free larva would have are the two means of attachment to the mother. 

 The first is the existence of recurved tips on the long strong chela? that, com- 

 bined with the instinct of the larva, enable it to seize hold of and become locked 

 to the material on the mother's pleopods. The second is the telson-thread that 

 holds the larva to the mother till it can get locked with its claws. This con- 

 trivance is formed in advance of its use both by early adhesions of membranes 

 and by later special secretions of telson glands so that its usefulness is depend- 

 ent upon a series of modifications of the ancestral history. 



In brief the first larval stage is embryonic in proportions and activities and 

 in lack of the perfection of sensory, feeding, locomotor, and defensive organs to 

 be expected in a free larva. It is actively specialized for its peculiar associa- 

 tion with the parent by the locking claws and telson thread. 



The second larval stage is transitional in structure from an embryonic to 

 a free larval state and in habit it remains still a fixed dependant in Cambarus 

 though in Astacus gradually becoming but loosely associated with the parent. 

 In both crayfish the second larva is still imperfect in lacking the sixth pleopods 

 and is embryonic in still containing yolk. It also has imperfections in sensory 

 and locomotor setae that may be interpreted as retrogressions from a more per- 

 fect ancestral state toward a yet more simple state like that of the first larva; 

 presumably the association with the mother was first acquired in the first stage 

 and has later extended, as yet, less thoroughly through the second stage. 



Supposing the ancestors of crayfish to have hatched as active larva the first 

 and second stages at present are new, in so far as they are specialized for at- 

 tachment to the mother and in that they are inactive and imperfect, while the 

 third stage may be compared more directly with ancestral larvae and is older 

 in its general structure. 



