XII] ASTACUS 193 



more waste than that of the worm. Some experiments which Dr 

 Eisig of Naples made on the Capitellidae, a family of Polychaeta, 

 seem to throw light on this subject. When he injected into these ani- 

 mals a coloured poison like indigo-carmine, a large part was excreted 

 by the nephridia, the cells of which became deeply coloured with the 

 material they extracted from the coelomic fluid. Part was however 

 got rid of by the skin, and was thrown off mixed with the substance 

 of the cuticle and was even incorporated with the chaetae. From 

 these facts Dr Eisig concluded that a portion of the waste products 

 of metabolism was eliminated through the skin. Now in the crayfish 

 the ectoderm consists of a single layer of pillar-shaped cells, which 

 are continually secreting at their outer ends the material called 

 c hi tin which contains nitrogen and is stated by some authorities to 

 be chemically allied to guanin and uric acid, substances which are in 

 many animals produced by the kidneys. As this secretion goes on 

 the shell increases in thickness, and confines the growth of the 

 animal. So at intervals this hard casing of dead matter is thrown 

 off: the old shell cracks along the mid-dorsal line, and the animal 

 gradually extricates itself, and the cuticular lining of the stomo- 

 daeum and proctodaeum is cast at the same time (Fig. 74). The 

 whole process is called ecdysis, and it occurs five or six times in the 

 first year, twice in the second year and after that once a year. After 

 each ecdysis the skin remains soft and growth can take place, but 

 of course during this period as the animal is defenceless it remains 

 hidden. Thus the tendency of the ectoderm to produce chitin has 

 not only governed the form which the locomotor and masticatory 

 organs have assumed but it has had the indirect effect of causing 

 the eoelom and nearly all the excretory organs to disappear, since 

 the skin has assumed most of the excretory functions. 



The genital organs have the same shape and position in both 

 males and females : they are situated beneath the pericardial septum 

 which is a sheet of connective tissue forming the floor of the peri- 

 cardium. In each sex the organ has a trilobed shape; there are 

 a pair of anterior lobes and a single median posterior lobe. It is 

 hollow and is connected with the exterior by a pair of ducts which 

 spring from the sides just where the paired anterior lobes pass into 

 the posterior one, and open to the exterior by a pair of pores 

 situated in the male on the coxopodite of the last pair of thoracic 

 legs, but in the female on the coxopodites of the third pair (in each 

 case counting the chela as the first leg). From the cells lining the 

 cavity of the gonad are produced the ova, which are about the size 



S. & M. 13 



