464 The Animal Kingdom 



and a posterior anus; often the alimentary tract is greatly coiled and 

 much longer than the body. Some forms have a closed circulatory sys- 

 tem ; all have nephridia which function as gonoducts. A pair of caeca, 

 the anal vesicles, which open into the rectum are the chief excretory 

 organs of the animals. These caeca are provided with ciliated funnels 

 which open into the coelom. The sexes may or may not be separate ; 

 at times sexual dimorphism is very marked. The main portion of the 

 nervous system is a ventral, unsegmented, nerve cord which gives off 

 many pairs of nerves. At the anterior end, this bifurcates and the two 

 resulting cords extend into and throughout the length of the proboscis. 

 There are no sense organs, but the proboscis is sensitive to touch. 



The members of the genera Echiurits and Urechis are found in 

 the coastal waters of this country. The genus Echiurits is the better 

 known one and is abundant in the North Atlantic and along the coastal 

 areas of Alaska; the genus Urechis is found along the Pacific Coast 

 from California to Chile, and in Japan. 



Both of these animals live in U-shaped burrows in mud flats and 

 keep a current of water flowing through by means of contractions of 

 their bodies. 



When feeding, Echiurus extends the proboscis from the burrow and 

 the outer edges pick up organic materials which may be scattered over 

 the mud's surface. This material is mixed with mucus and carried to 

 the mouth. The proboscis is then withdrawn and protruded in an- 

 other direction. Urechis has a totally different method of feeding. 

 It constructs a mucus tube at the top of its burrow then slowly with- 

 draws, leaving its upper portion attached to the bottom of this tube. 

 By a series of pumping motions, water is rapidly drawn through the 

 tube and food materials are caught. When the tube is filled with food, 

 Urechis contracts to loosen the tube from its body, grasps the tube 

 with its proboscis, and swallows it. A new tube is usually spun im- 

 mediately. 



These animals seldom need to make new burrows and may occupy 

 a single site for many years. As a result, they often have commensals 

 dwelling with them. These are such animals as small crabs, clams, 

 small fish which benefit from the constant currents of aerated water and 

 food. 



Neither of these genera possesses special gonads ; the eggs and 

 sperm are budded off from the body cavity lining. They are usually 

 stored temporarily in paired sacs within the coelom. Urechis spawns 



