ASSOCIATIONS 



603 



itself by its antennae. Fixation accomplished, the internal contents of the 

 cypris contract into a cellular mass and the animal sheds its integument 

 together with its appendages. A thin chitinous dart is then formed, which 

 pierces the body wall of the crab, and through this opening the cellular 

 mass of the cypris invades the visceral cavity of the host. Development is 

 slow, occupying twelve months, and during that period the Sacculina 

 migrates from the point of penetration along the intestine. It takes on the 

 appearance of a mass of branching rootlets and, on arriving at the unpaired 



Fig. 14.14. Sacculina, a Rhizocephalan Parasite of Crabs 



(a) Nauplius larva; (b) cypris larva; (c) section through external mass of Sacculina; 

 (d), (e) and (/) growth of parasite internally along the intestine of the crab, ((a), {b), (d), 

 (e) and (/), after Smith (94); (c) after Delage, 1884.) 



intestinal caecum, it gives rise to a tumour-like nucleus, which develops 

 into the saccular structure visible externally. On reaching the outer wall, 

 the nucleus causes necrosis of the host's tissues and softening of the chitin, 

 thus permitting access to the exterior (94). 



Well-known rhizocephalan forms are Sacculina parasitic on Carcinus, 

 Portunus and Inachus ; Peltogaster on pagurids ; Parthenopea on Gebia and 

 Callianassa; and Thompsonia on Melia, Thalamita and Alpheus. In an 

 infection with Thompsonia up to 200 reproductive sacs may appear on the 

 outside of the prawn, and these arise from a common system of rootlets 

 due to infection by a single cypris larva (80, 104). 



