Handbook of Paleontology 111 



arms may be broken off piece by piece until only the 

 disc remains. In sea cucumbers autotomy involves 

 the internal organs and is a process of self-eviscera- 

 tion. This happens when the animal is strongly irri- 

 tated and may involve part or the whole of the viscera, 

 regeneration taking place in a fairly short time. There 

 are other miscellaneous cases of regeneration but these 

 are among the most striking. 



Through the crowded conditions along the shore an- 

 other type of struggle is brought about. There is a ten- 

 dency for many organisms to live on the bodies of others. 

 Small seaweeds grow upon the shells of slow-moving gas- 

 tropods such as Limpets and Periwinkles. Various sea- 

 weeds are also found attached to the species of the Shore 

 Urchin {Echinus) ; and one of the green seaweeds {En- 

 ter omorpha) often marks with its tufts the position of 

 Cockles in the sand. The broad fronds of seaweeds are 

 of advantage to incrusting animals. The empty air vesi- 

 cles of the brown seaweed Ascophyllum house the larvae 

 of the Common Mussel {Mytilus edulis). Sessile forms 

 of animals such as hydroids, bryozoans and barnacles, 

 settle upon the bodies of other animals just as they settle 

 upon rock. Mollusk shells, particularly the sessile bi- 

 valves such as oysters, may be heavily incrust'ed not only 

 with other forms but with the young of the same species. 

 The Slipper or Boat Shell Snail {Crepidida) attaches 

 itself to other shells or to shells of the same species and 

 five individuals have been found occurring one on top of 

 the other. Slow moving crustaceans carry a variety of 

 guests, among them algae and hydroids which may be 

 of advantage in that they serve to conceal the host. The 

 higher crustaceans are liable to be incrusted with seden- 

 tary forms, especially barnacles, until the load becomes 

 embarrassing, and molting alone frees the animal from 



