of sediments. The feeding tentacles are fleshy, some- 

 times with a few finger-hice extensions at the ends. 



According to Japanese scientists, molpadonias 

 feed particularly rapidly. An individual may move 

 from 125 to 150 pounds of bottom sediments through 

 its 7-inch body annually in extracting nourishment. 

 One of this type of cucumber is Caudimi arenata, 

 found from Rhode Island to the Gulf of St. Lawrence 

 between 100 feet below the surface and low-tide 

 mark. Its tail tip can be found exposed from the sandy 

 mud, and used to capture the buried cylindrical ani- 

 mal. The body may be 1 inch in diameter and 7 in 

 length, in hues ranging from deep purple to flesh- 

 color. 



Some sea cucumbers are wormlike, lacking tube- 

 feet and respiratory trees. Usually the body wall is 

 very thin, often translucent, and the animal itself is 

 more active than most other holothurians. Several 

 kinds with this shape of body burrow in the mud and 

 can bury themselves in five to six minutes. Others, 

 while only partly grown, swim to the surface at night 

 by a curious twitching movement of the body, sug- 

 gesting a scissors kick. 



Synaptida is one of the commoner wormlike sea 

 cucumbers. It can be found clambering among sea- 

 weeds and through coral reefs. When fully extended 

 a Synaptida may reach a length of 3 feet, yet be 

 no more than V2 of an inch in diameter. Some mem- 

 bers of this genus have openings through the wall of 

 the intestine in the female, through which sperms 

 from sea water reach the eggs in the body cavity. 

 Thus fertilization is internal, and the embryos develop 

 for some time in the body cavity before being cast 

 out into the sea. The young of Chiiidota rotifera, a 

 wormlike sea cucumber of shallow water in the West 

 Indies, reach the same body form as the parent be- 

 fore they emerge, and this sea cucumber is truly 

 viviparous. 



Sea Stars 



(Class Asteroidea) 



For many people, a starfish, better called a sea star, 

 is a clear symbol of marine life (Plates 118-130). 

 They are aware that animals of this type are never 

 found in fresh water or on land. Sea stars are strictly 

 bottom animals, chiefly of the margins of the sea. 

 They range in size from less than Vi of an inch in 

 diameter to more than 3 feet across, and in shape 

 from regular stars with five or more arms to pentago- 

 nal and almost circular. Yellow, orange, pink, or red 

 are the commonest colors, but gray, green, blue, and 

 purple ones can be found. 



Most sea stars take mollusks as their favorite food, 

 but some eat sea urchins, sea cucumbers, and other 

 sea stars. A few catch small fish and shrimps. Perhaps 

 a majority will devour carrion on the bottom. Sea 



stars of certain kinds swallow soft mud and digest 

 the organic matter, as do so many other echinoderms. 



A sea star's arms are actually part of its body 

 rather than appendages. If one of these animals is 

 turned upside down to expose its mouth surface, each 

 arm is seen to have a lengthwise groove filled with 

 moving tube-feet. Within each arm the star has also 

 one or two branches of its reproductive organs, and 

 often extensions from the digestive tract as well. 



If the inverted sea star is balanced for a few sec- 

 onds on the back of a person's wrist, it may take hold 

 firmly enough to hang from the human hairs after the 

 wrist is turned over. Its grip is not strong enough to 

 pull out the hairs, but once attached in this way, it sel- 

 dom will let go. 



This trick is not due to tube-feet, for stars have 

 none of these organs on the aboral surface. Instead, it 

 is a demonstration of strange little modified spines 

 (pedicellariae) by means of which the animal ordi- 

 narily polices its body. Each pedicellaria commonly 

 has the form of a pair of pincers, or of a little clam 

 shell lying in a minute pocket of the surface. The 

 parts open or close under muscular control. Sea ur- 

 chins are the only other animals in the world with 

 pedicellariae. 



Between the pedicellariae over much of the star's 

 aboral surface, small domes of soft skin show where 

 the body cavity is separated from the sea by only a 

 thin layer of tissue. Here the blood can exchange car- 

 bon dioxide for oxygen. Elsewhere the outer surface 

 of the star, like the lining of its digestive tract and the 

 corresponding parts of sea urchins, crinoids, and brit- 

 tle stars, is covered by cells with cilia. These propel a 

 thin film of mucus from glands and, on the outside of 

 the body, efficiently keep it clean by sweeping away 

 any debris that falls on it. 



One or more madreporites, flat plates perforated 

 by many holes, can be found on the aboral surface of 

 a sea star. Through these the water-vascular system is 

 connected to the ocean. Most sea stars have only one 

 madreporite, and naturalists have often used this land- 

 mark as a guide in trying to learn whether a sea star is 

 actually so versatile that it will travel with equal readi- 

 ness in all directions, letting any arm lead the way. 

 Some few kinds and occasional individuals seem to 

 show a distinct preference. For the rest, the radial 

 plan extends to habits, and all arms are equivalent. 



When a star rights itself after falling on the bottom 

 with mouth up, it shows little partiality for one way or 

 another. Yet for a patient observer, the star's move- 

 ments provide a wonderful demonstration of its re- 

 markable flexibility and muscular control. The crea- 

 ture may take anywhere from two to ninety minutes 

 to get back on its tube-feet. 



Some stars turn over by use of the "tulip method." 

 Slowly they bend all of their arms in the same direc- 

 tion, perhaps raising them around the mouth like the 



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