144 COMPARATIVE ANATOMY 



assumes the distinctive characters of the adult limb. Thus 

 the first and second antennae soon become biramous ; the 

 mandibles are never biramous, but at an early stage show 

 a differentiation into a thickened proximal joint, and a conical 

 appendage which will become the jointed palp. In the first 

 maxillae the endopodite is from the first predominant, the 

 exopodite being represented only by a small swelling. The 

 second maxillae and the maxillipeds, which have both 

 endopodite and exopodite in the adult, are from the first 

 biramous. The chelae and first two pairs of pareiopods seem 

 to be biramous at an early- stage ; the terminal branches, 

 however, do not represent endopodite and exopodite, but 

 the rudiments of the chelae characteristic of the adult limbs. 

 The abdominal limbs are biramous from the first. In short, 

 the limbs grow directly from mere buds into their permanent 

 form without passing through any ancestral stages. The 

 growth of the thoraco - abdominal swelling requires close 

 attention. It projects more and more from the surface of 

 the egg, and as it grows it is folded down beneath the anterior 

 part of the embryo, so that its extremity nearly touches the 

 labrum. It is divided by a series of constrictions into eleven 

 segments namely, five thoracic bearing the rudiments of 

 the chelae and the four pairs of pareiopods, and six abdominal 

 bearing the six pairs of .pleopods. At the end of all is the 

 terminal piece or telson, divided by a deep median incision 

 into two lobes. As the telson increases in size, the anus is 

 shifted from its original posterior position to the ventral surface. 

 While these changes are in progress, the eyes grow out 

 as two conspicuous prominences in front of the first antennae. 

 Beneath them a pair of thickenings of the ectoderm gives 

 rise to the optic ganglia, and a little further back a similar 

 pair of thickenings is formed in connection with the first 

 antennae, and yet another in connection with the second 

 antennae. These three pairs of thickenings eventually fuse to 

 form the cerebral ganglion, and it is interesting to note that 

 the " brain " of the crayfish is shown by its development to 

 be a syncerebrum, formed by the fusion of the optic and 

 two succeeding pairs of ganglia appropriate to the appendages. 

 The thoracic and abdominal ganglia are formed from 

 ectodermic thickenings in connection with the successive 

 pairs of limbs. 



