no A Guide to the Zoological Collections in the 



things that is 'never found in parasitic animals, which always 

 show a degradation of some or all of these organs. 



The fish and crab are really "commensals^' or mess- 

 mates with the Holothurians with which they live. " This 

 living together of two animals is known as * symbiosis '. It 

 differs essentially from parasitism, in which one organism 

 preys upon another, the host deriving no benefit but only 

 harm from the presence of the parasite. In symbiosis on 

 the contrary the two organisms are in a condition of mu- 

 tually beneficial partnership". — (Jeffery Parker). The 

 Holothurian furnishes a safe retreat for the fish and the 

 crab, while the fish and the crab probably serve the 

 Holothurian by keeping it clean in some way. 



[Another excellent instance of symbiosis is illustrated 

 in Case i6, where the coralla of Heterocyathus are found 

 to be tenanted each by a small worm. In this case the 

 Heterocyathus serves the worm by affording it a safe re- 

 treat, while the worm returns the service by carrying the 

 Heterocyathus about from one feeding ground to another, 

 thus transforming it from a fixed and sedentary into a free- 

 ly locomotive animal]. 



The Holothuridea are found at the bottom of all seas 

 from the surf line to the greatest depths of the ocean. 

 They live, as is well illustrated in the dissected speci- 

 mens, upon sand and mud, with which they gorge them- 

 selves for the sake of the small amount of organic matter 

 (remains of dead animals, living animalcules, etc.) that 

 is contained in its interstices, very much in fact as earth- 

 worms do. 



The Holothiiroidea are represented in the exhibited col- 

 lection by the following genera from the Indian area: — 



(i) Deep-sea forms — Deinia, Orphnurgus, Pannychia, 

 Laetmogone, Euphronides, Psychropotes, Benthodytes, 

 Apodogaster ; Eupyrgus, Ankyroderma, Trochostoma : 



{2^ Shallow -water and Reef forms — Synapta, Haplodacty- 

 lus Caudina ; Cucumaria, Colochirus, Ocnus, Thyone, 

 Thyonidium, Stickopus, Holoihuria, Bohadschia, Mulleria. 



