CRUSTACEA. 



337 



Other parts appear in pairs ; the antennae (Jb, c), at first short and 

 simple processes, increase in length, and their extremities become 

 notched ; the mandibles (d) also lengthen and enlarge, particularly 

 in their basal portion ; the labrum recedes from between the anterior 

 antennae, and takes its station between the posterior : the eyes (a) 

 now make their appearance. All these parts are, at first, slight 

 elevations above the surface of the germ. A cavity is formed behind 

 the labrum, which communicates with the commencement of the oeso- 

 phagus ; the tail or abdomen (i) elongates, and the depression in its 

 surface is converted into the anus ; the rest of the alimentary canal 

 is a simple wide sac, which, by the extension of the mucous layer of 

 the germinal membrane, now includes the vitelline mass. 



The three anterior pairs of maxillse begin to show themselves at a 

 little distance behind the mandibles, and afterwards the fourth and 

 fifth pairs ; the last {Jiff. 137, ff 5) increasing in size more rapidly 

 than the rest. Thus, including the 

 eyes (a) and the two antennae (6, c), nine 

 pairs of appendages may now be recog- 

 nized, of which the two last belong to 

 the thorax : the five posterior pairs of 

 thoracic members, Avhich are not, like 

 the first two, developed into jaws, are 

 produced in regular succession from be- 

 fore backwards from that portion of the 

 body which is turned upwards, or the 

 epimeral elements of the rudimental 

 segments. Each pair of legs at its first 

 appearance is exactly similar to the hindermost maxillse ; these, there- 

 fore, are retained in the service of manducation by an arrest of develop- 

 ment. The ambulatory legs increase inversely with respect to the 

 maxilla, the anterior (Jiff. 137, k 1) soon acquiring four times the size 

 of the posterior {h 5). The rudiments of the future branchiaj next 

 appear as small processes from the base of each leg. The seven seg- 

 ments of the third division of the body may now be distinguished by 

 six transverse furrows, and by the rudiments of foliaceous appen- 

 dages. Part of the cephalic segment (m) above the antennae is the 

 basis of the great shield which afterwards covers the whole cephalo- 

 thorax ; it grows upwards and backwards until it meets its fellow 

 along the median line of the dorsal aspect, and so completes the 

 carapace. 



The heart (s) appears at first in the shape of a small compressed 

 vesicle, or "punctum saliens," situated near the junction of the 



z 



Astacus fluviatilis. 



