COMMON CRA YFISH. 



167 



in 10 days' time; then four times at 20-25 days' interval up to September. Three 

 more moults in May-July following complete the first year of life. There are 

 five in the second year, two in the third, and the animal is now becoming adult. 

 The male is sexually mature after the ^th-iyth moult, and henceforward under- 

 goes ecdysis twice yearly. The female is sexually mature in the fourth year, 

 and only moults once a year. The process is very probably in abeyance in old 

 Crayfish, but it is not certain at what age they cease to grow. According to 

 Carbonnier there are 2-3 moults in the first year, and then the process becomes 

 annual. 



When the time approaches for a moult, the old integument becomes softer and 

 thinner, and the ectoderm cells lengthen. The gastroliths and hairs are previously 

 formed. The first step, according to Braun, is the formation of minute processes, 

 2-5 to each cell, which afterwards form (?) the ridges of the areae (see supra]. Vitzou 

 did not observe this formation in any Decapod, but states that the inner surface of 

 the cast-off integument is covered by a soft transparent substance. A section taken 

 through the old and new integuments shows the presence of a new and very delicate 

 cuticle, an outer system of continuous lamellae, and an inner system traversed by 

 vertical lines, corresponding to the contours of the subjacent ectoderm cells. 

 Vitzou concludes from this fact, and from the subsequent shortening of the 

 ectoderm cells, that the lamellae are developed by a specialisation, chemical and 

 physical, of the outer ends of these cells. And his opinion is borne out by the fact 

 that in sections taken parallel to the surface from the carapace of Crabs polygonal 

 areae corresponding to the ectoderm cells may always be seen. The outlines are 

 brought more distinctly into view by treatment with silver nitrate. But in most 

 instances, e. g. Lobster, these areae are obliterated by early fusion of the cell-walls. 

 Tullberg appears also to be of the opinion that a conversion of the outer ends of the 

 cells takes place during the formation of the new carapace (cf. Zoolog. Jahresbericht, 

 1882, Arthropoda, p. 12). Vitzou has proved the presence of glycogen in the con- 

 nective tissues preparatory to and during the moult. Certain tendons, the cornea of 

 the eye and the lining membrane with the hairs of the auditory sac, are regenerated 

 at the same time as the rest of the integument. The endophragmal skeleton is 

 broken up previously to being cast off (Mocquard, C. R. xcvi.). The new cuticle 

 of the alimentary canal is formed at a comparatively late period. 



The pores in the lamellae are due to the development of hair-like processes from 

 the cell-surfaces. They are very close-set, and subsequently disappear leaving the 

 pores (Braun). 



The old integument splits in all Macrura across the back, between the cephalo- 

 thorax and the first abdominal somite. A split is said to occur also along the 

 limbs. The animal lies upon its side and withdraws first the cephalothorax and 

 then the abdomen with their several appendages. In the Crabs a split occurs along 

 a circular suture between the tergal and pleural regions of the cephalothorax. The 

 animal retains the horizontal position, and the abdomen is freed before the cephalo- 

 thorax (Vitzou). 



The appendages in front of the mouth, and those which are modified into 

 mouth-parts must be disarticulated for the purpose of examination. 



The eye-stalk has a short basal and a long terminal joint. It was formerly sup- 

 posed that it represents a limb, and therefore a somite. It is however derived 



