SKELETONS, SHELTERS AND SPECIAL DEFENCES 663 



prawn living in coral, and Caphyra, a crab inhabiting depressions in 

 Alcyonium. 



Certain operculate cirripedes (Creusia, Pyrgoma) are confined to living 

 corals. Different species of Pyrgoma display host specificity and are found 

 on particular reef or deep-water corals. The growth of these cirripedes is 

 correlated with that of the host. Another commensal of madreporarian 

 coral is the gastropod Magilus antiquus, which lengthens the last convolution 

 of its shell in a linear fashion, and thus keeps pace with outward growth 

 of the coral (55, 56,98). 



ADVENTITIOUS SHELTERS 



Commensal relationships are very common among marine animals, and 

 there are many instances of animals seeking shelter in the burrow and tubes 

 of their rightful owners (endoecism), or even in body cavities of other 

 species (inquilinism). Intimate associations of this nature between different 

 species are described in Chapter 14. 



Empty tubes, shells and other skeletal structures are frequently utilized 

 for shelter by marine animals, especially crustaceans. Some sea-urchins, 

 e.g. Psammechinus, have the habit of covering themselves with shells, 

 pebbles, etc. There are holothurians, e.g. Pseudostichopus, which cover 

 themselves with siliceous sponge spicules. Some brachyurans employ 

 shields for protection and defence. Dorippe has the last two pairs of legs 

 modified for holding objects over the back, and tropical species carry 

 around molluscan shells, mangrove leaves, etc. 



The habit of dwelling in empty gastropod shells is widespread among 

 hermit crabs. When young the crabs select small shells and discard these 

 for larger shells as they grow in size. In conformity with this habit we find 

 that hermit crabs usually have twisted abdomens which can fit into coiled 

 gastropod shells, e.g. Eupagurus. Terrestrial hermit crabs Coenobita 

 procure shells from the shore, but on occasion make use of other structures, 

 e.g. coco-nut shells. These pagurids have soft unarmoured abdomens. The 

 terrestrial robber-crab Birgus has abandoned the habit of carrying a shell 

 about, and the abdomen has secondarily acquired hard terga. 



Pomatochelids are a family of primitive hermit crabs possessing 

 symmetrical abdomens. Shelters utilized include Dentalium shells 

 (Pomatocheles) or water-logged tubes of bamboo and mangrove 

 (Pylocheles). 



A curious example of the utilization of another animal's covering is 

 afforded by the pelagic amphipod Phronima. This animal inhabits the 

 hyaline barrel-like cases of Pyrosoma and Doliolum. The female Phronima 

 attacks the Pyrosoma or salp and, after eating the ascidiozooids, continues 

 to dwell in the test. Phronima is able to navigate its house: holding on to 

 the case by the thoracic legs, it protrudes the rear portion of the body and 

 propels the case forwards by alternate flexion and extension of the ab- 

 domen. Water drawn in at the front end of the case provides a feeding and 

 respiratory current for female and young. 



