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Part III. — Eighteenth Annual Report 



animal. If the tips of the large chelipeds are clipped off, autotomy does 

 not always or usually occur, and the limb is completely repaired after 

 one moult." Couch* in this connection said — " I am led to believe 

 that when a limb has been lost at some moderate time previous to 

 exuviation, the restoration is much more rapid and perfect at the next 

 succeeding- time of the process than if left to the more ordinary pro- 

 ceeding of nature." 



The crab can throw off an injured limb, or it may when seized by a 

 foe, in order to escape — purchase freedom at the voluntary expense of a 

 limb. It is then severed at what is known as the fracture-plane, which 

 is marked externally by a suture, extending round the second or 

 basi-ischiopodite joint. Within the segment there are at this point two 

 closely-applied diaphragms which, except for a small foramen, shut off 

 the short proximal from the longer distal part of the limb. The 

 foramen piercing the two diaphragms gives passage to the nerves and 

 vessels that supply the appendage. f (Plate I., Fig. 7.) The fracture 

 takes place between the two septa, the proximal diaphragm remaining 

 as a covering to the broken second joint, the other going with the cast- 

 off limb. The foramen is immediately closed with a small clot of blood. 

 The edge of the shell and the diaphragm soon turn black ; a new skin 

 develops within, and the outer black layer, which is now dead tissue, is 

 gradually rubbed off, exposing the new skin. Within this skin, when 

 the crab is preparing to cast, a new limb is formed, and, as it grows, 

 bulges the external covering out in the form of a bud. The scars which 

 mark the lost limbs are found in many different stages — (1) Immedi- 

 ately the limb is lost the scar is white ; (2) then the outer skin (the 

 diaphragm) dies and becomes black — this is accompanied by the forma- 

 tion of a new skin beneath it ; (3) the black dead tissue is rubbed off 

 through time, or on the bulging out consequent on the formation of 

 the bud ; (4) the live skin, when exposed by the removal of the dead 

 tissue, becomes brown. The formation of a bud soon after the limb is 

 lost, and while the dead tissue still covers the scar, is dependent on the 

 nearness of the casting period. 



These conclusions are founded upon the observations made upon the 

 beach crabs during each month for a year. Buds of new limbs were 

 noticed mainly during the casting period — viz., June (4 individuals), 

 July (21 individuals), August (10 individuals), and September (3 

 individuals). In the case of very small crabs, less than 1 inch across, 

 the papilla which is the rudiment of the new limb has been observed 

 in September, October, December, January, and February ; while buds 

 have been noticed in November, December, February, March, May, and 

 July. Two crabs measuring 3 inches and 1| inches respectively shewed 

 in December and January small papillae on the scars of lost limbs. 

 The crabs under 1 inch in breadth doubtless cast more than once 

 in the year. The only crab which was found soft on the beach 

 in February was one measuring i inch. It had just cast. I have 

 no data bearing upon the time required for the formation of the 

 new limb, but it appears that only in the case of a limb lost 

 some considerable time before casting will regeneration be com- 

 pleted at that time. Thus, a crab measuring If inches across cast 

 in a tank at Dunbar in September. Two of its limbs were missing, 

 and a third was injured. The dactylopodite of one biting claw had been 

 broken off, and the injury was covered by a black scar. The soft crab 

 was without three limbs, and had both chelae perfect. Two soft crabs 



* Op. cit., p. 11. 



f L. Fredericq gives a detailed description and figures of this part of the limb in C. 

 pagtirus with reference to the phenomena of autotomy. He describes, however, only one 

 diaphragm, his "membrane obtenatrice " corresponding to the proximal of the two 

 described above. "Nouv. Rech. s. l'autotomie chez le crabe," Arch. Biol., XII., 1892, 

 p. 177 et se%. 



