PARASITES AND MESSMATES 231 



fully formed it bores its way through the tissues of 

 its host by means of rows of hook-like spines 

 surrounding the pointed posterior end of the sac. 

 On reaching the surface the enclosing membrane 

 bursts, and the adult animal is set free. 



Of all Crustacean parasites, however, perhaps the 

 most remarkable in their structure and life-history 

 are the Cirripedes of the order Rhizocephala. It is 

 not uncommon on the British coasts to find specimens 

 of the common Shore Crab or other Crabs which 

 carry under the abdomen an oval fleshy body. This 

 is the Rhizocephalan Sacculina carcini (Plate XXIX.), 

 and it would hardly be possible to guess, from its 

 appearance or structure, that it was a Cirripede or a 

 Crustacean at all. It is attached to the under-side 

 of the Crab's abdomen by a short stalk, and in the 

 middle of its opposite surface is a small opening 

 which leads into a cavity separating the outer 

 " mantle " from the body of the animal. Very often 

 this mantle cavity will be found to be full of eggs 

 enclosed in sausage-shaped packets. At the point 

 where the short stalk enters the abdomen of the 

 Crab, it gives off an immense system of fine branching 

 roots, which penetrate throughout the body of the 

 Crab, and even into its legs and other appendages. 

 By means of these roots the Sacculina absorbs 

 nourishment from the body-fluids of its host. Like 

 most Cirripedes, Sacculina is hermaphrodite, and the 

 body within the mantle cavity contains only the 



