[conlinuci.1 rrom page 256] 



Sea Cucumbers 



{Class Ho loth mo idea ) 



A person need wade out only knee-deep around 

 many of the Florida keys to encounter, lying con- 

 spicuously exposed on the muddy bottom, large sau- 

 sage-shaped creatures 1 foot or more in length and 

 better than 2 inches in diameter. Against the pale 

 gray surface on which they lie, the contrast may be 

 striking: dark brown with light spots, or brick red 

 with raised lumps of black or dark brown. They are 

 sea cucumbers (Plates 140, 141 ), with a name given 

 them {Ciiciiinis niariniis) in the first century a.d. by 

 Pliny, the Roman elder and encyclopedist. 



Upon closer examination, none of the usual clues 

 is evident to show which is the head end of the ani- 

 mal. As it lies there quietly, both ends of the cucum- 

 ber appear to be doing something. At one end. an 

 opening appears, sometimes as much as 1 inch in 

 diameter. If the water is shallow, a current may be 

 noted pouring out of the opening. Or the sea cucum- 

 ber may be taking in water just as rapidly. The move- 

 ments of the opening and the slow enlargements and 

 contractions of the whole body suggest a sort of 

 underwater whistling. Actually, they are breathing 

 movements and, in this unusual animal, occur at its 

 rear end. 



As though further to astonish the beachcomber on 

 tropical and subtropical shores, a sea cucumber may 

 be found from which a fish's head projects. The fish 

 is very much alive, and the cucumber's breathing 

 movements simply take in water or expel it around 

 the fish. If nudged, the fish may swim out, exposing 

 a slender tapering body as much as six inches long. 



Usually a minor drama follows at once. The 

 blenny-like fish turns back immediately to the side 

 of the sea cucumber and moves about along the sur- 

 face, evidently searching for the respiratory opening 

 again. Often the sea cucumber closes the aperture 

 tightly, as though to keep the fish from returning to 

 its refuge. But eventually the need for oxygen be- 

 comes too great. The cucumber opens again, the fish 

 slips in either tail first or head first (and turns around 

 immediately ) . 



The cavity into which the fish goes is the cloaca of 

 the sea cucumber, a chamber serving not only respira- 

 tion but also as a common exit for wastes from the 

 digestive tract and sex cells from the reproductive 

 system. Sea cucumbers are unique in having a pair 

 of generously branched "respiratory trees" extending 

 blindly from the cloaca far forward in the body cavity. 

 Through their walls oxygen and water pass, keeping 

 the other internal organs aerated and maintaining the 

 plumpness of the cucumber's body. 



At the opposite end of the animal a set of tentacles 

 moves slowly, obtaining food. In most sea cucum- 



bers, including the large kinds found near shore in 

 tropical and subtropical waters, these soft organs 

 around the mouth shovel the surface mud into the 

 digestive tract, letting the animal get the nourish- 

 ment from a great assortment of microscopic life, 

 especially diatoms. The gritty residue is expelled from 

 the cloaca, and sometimes accumulates into conspicu- 

 ous heaps. The late Professor W. J. Crozier estimated 

 from measurements of the cones of debris that the 

 sea cucumbers on each acre of bottom in one region 

 ofl' Bermuda would pass between 100 and 200 

 pounds of sand through their bodies annually. 



Substantial amounts of the nourishment obtained 

 by a sea cucumber are stored in its body wall. There 

 the food reserve usually gains protection from a 

 slimy, leathery skin in which are embedded little 

 hmy secretions of remarkable variety. Some are mi- 

 croscopic plates perforated by many holes. Others are 

 knobby rods, or anchor-shaped, or resembling a con- 

 crete bird bath or a wheel with spokes but no rim. 

 Each species has its own distinctive limy granules. 

 Only a few kinds lack them altogether. 



Many of the larger sea cucumbers that live close 

 to shore supposedly discourage attack by fish and 

 crabs through the presence of a poison (holothurin) 

 in their skins. If extracts of it are injected into mice, 

 they die quickly. The presence of certain sea cucum- 

 bers in an aquarium tank may be enough to poison 

 any fish present. With some of the large subtropical 

 and tropical cucumbers, the effect sometimes persists 

 in a tank for weeks after the echinoderm has been 

 removed and the water changed repeatedly. 



Large cucumbers belonging to the genera Holo- 

 thiiiia ( Plate 141 ) and Actinopyga have ready a truly 

 astonishing defense against animals that molest them. 

 Associated with the region where their respiratory 

 trees open into the cloaca they have short tubules of 

 red, pink, or white color. If the echinoderm is dis- 

 turbed seriously or repeatedly, it slowly turns its body 

 until the cloacal opening faces the molester, then 

 performs a general contraction and proceeds to send 

 out the slender tubules in great numbers. The blind 

 ends of the tubules may be enlarged; almost always 

 they are very sticky. And as they emerge from the 

 cloacal opening, they become darting, adhesive 

 threads that, in a minute or less, can so enmesh a 

 crab or lobster that it is immobilized. The cucumber 

 frees itself from the tubules and moves slowly away 

 as though nothing had happened. 



With provocation these and many related sea cu- 

 cumbers will perform a far more amazing trick. With 

 a single powerful contraction they turn themselves 

 partly inside out — throwing out the respiratory trees, 

 the reproductive organs, and sometimes some of the 

 intestine as well. All of these emerge suddenly through 

 the cloacal opening as a tangled mass over and around 

 a crab or fish. From these too the cucumber separates 



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