THE YOUNG OF THE CRAYFISHES ASTACUS AND CAMBARUS 19 



figures of the English Astacus, which, however, were doubtless drawn from 

 alcoholic specimens, and in A leniusculus the action of alcohol is to cause great 

 distortion of the branchiostegites. As compared with the adult, however, the 

 proportions of the head-thorax are embryonic and there must needs l)e much 

 greater elongation as well as lateral and vei-tical changes to bring about adult 

 proportions. 



While both from side and top views (figs. 3 and 4), the characteristic 

 rostral sjoine of such Crustacea seems absent, full front and diagoual side 

 views (fig. 5) show the rostrum to be well developed and armed with lateral 

 spines, but so bent down between the eyes as to be of no such use as a de- 

 fense as it later will be in active stages of the larva. The habit of the first 

 stage which clings to the parent is thus correlated with imperfections of defen- 

 sive armament as well as with presence of food yolk and imi)erfections of loco- 

 motor organs. Among the latter characters may be reckoned the smooth sur- 

 faces of the limbs and absence of setae that later will increase the areas of 

 resistance for striking against the water as well as furnish means of sense 

 perception. The lack of seta; represented in figures 3 and 4 is still more strik- 

 ing in enlarged views of the limbs and is in strong contrast to the hirsute char- 

 acter of all parts of the active larvie and of the adults, and this absence of 

 seta; gives the larva^ a decidedly embryonic appearance. 



The pronounced incompleteness of locomotor organs is also associated with 

 the shortness of the thoracic region bearing legs ; thus the cheliB arise farther 

 from the anterior than from the posterior ends of the head-thorax and leave 

 little space for the walking legs, while the anterior region containing the yolk is 

 greatly developed in size. 



Next taking up in sequence the nineteen pairs of appendages of the adult 

 we find them represented in the first larva by seventeen pairs that have in the 

 gross, as made out by Rathke for embryos about to hatch, the essential mor- 

 phology of the adult appendages but lack the setae and differ in proportion as 

 will be seen from the following account and illustrations. 



The whole exterior of the larva in its first stage is covered by a chitinous 

 exoskeleton of such resistance that when the young were thrown into Wor- 

 cester's liquid they did not die for several minutes; the appendages were cut 

 off separately from such hardened embryos and gave the views represented in 

 the following illustrations. 



The first antenna stands out horizontally in front of the head (figs. 3, 4) 

 and is straight ; as seen in figure 6, it has three basal segments, five in exopo- 

 dite and in endopodite. The segmentation of the endopodite is very obscure. 

 The terminal segment in both endopodite and exopodite bears three obscurely 

 pointed spines, one of which, in the exopodite, is long and apparently of the 

 same character as the sensory setae found there in later stages. On the long 



