THE YOUNG OF THE CRAYFISHES ASTACUS AND CAMBARUS 57 



as the larva was now well able to follow its strong instinct to crawl upward 

 and so recover its position on the mother, or, failing that, to live independ- 

 ently, as it soon did. 



In the following description of the third larva we will emphasize its man- 

 ner of association with the parent. 



In general form the young crayfish in this third stage (figs. 90, 91) is now 

 for the first time like the adult, yet the great size of the eyes and of the ex- 

 opodite scales of the antennae together with the very wide expanse of the tail 

 fan and its long swimming plumes give the larva the aspect of a pelagic larva 

 or of the adult of some lower form of marine decapod. In fact the ability to 

 swim freely in great leaps which the third larva soon exercises and the trans- 

 lucency of the body together with the big eyes and long antennae and very 

 slender long legs and chelae make the larva seem much like a small shrimp. 



The rostrum is now directed straight forward and is armed with large 

 lateral spines that exaggerate the gothic style of the adult. All the appendages 

 are now provided with long setae which make them sparsely hirsute as seen 

 under the microscope, but only on the exopodite scale of the antenna and upon 

 the telson and sixth pleopods are the setae set in rows that suggest a locomotor 

 function. The use of the row of plumes on the scale of the antenna is not 

 obvious but the long plumes all along the edge of the telson fan seem to add to 

 its area effective in locomotion. 



The animal was still translucent and within it the liver showed as a narrow 

 green band external to the large mandibular muscle, extending forward a short 

 distance and backward nearly to meet its fellow in the region of the heart. 



The sixth pleopods were now no longer inside the telson and the telson 

 had become specialized as in the adult into an anterior and posterior part 

 separated by a distinct movable hinge. Through the clear dorral exoskeleton 

 was seen the anus and a large mass of muscle on each side in the anterior 

 region and in the posterior part of the telson the old radiating glands, which 

 had eventually perfected the long setae that a high power showed to be plumose. 



The pigment cells represented by dark dots in figures 90 and 91, had be- 

 come more numerous than before but were still chiefly arborescent red cells, 

 though in many regions there were large blue cells not represented in the above 

 figures though they now altered the ground tone of the animals. Posterior to 

 the eyes the crowded pigment cells formed a streak along the base of the lateral 

 spines and more to the rear a rounded area over the attachment of the great 

 mandibular muscle. This round ai'ea on each side was especially conspicuous 

 from the dense crowding there of blue pigment mixed with red. The dark 

 aggregation of pigment cells across the anterior edge of the abdomen still re- 

 mained. The yolk area and its color were quite gone and on each side of 

 the stomach a blue area indicated the gastrolith. Owing to the fineness of the 



