ECHINODERMATA 10} 



bodv, though it often occurs at the extremity of the sti mal, 



which lies freely within the body-cavity. 



The alimentary canal is much longer than the chief axis of 

 the both' and lies coiled up in the body-cavity, its walls usually 

 having a rich supply of blood vessels, its posterior end is 

 enlarged, forming a saclike chamber called the cloaca, into 

 which open two greatly branched systems of tubes, which gen- 

 erally extend throughout the whole length of the body-cavity, 

 and are called the respirator} - tree because of their supposed 

 function and their treelike appearance ( Fig. 94). There is a 

 single germ-gland, which lies in the dorsal interradius, i.e. 

 between the two ambulacral areas called the bivium ; it con- 

 sists of a number of tubules, whose common duct passes forward 

 and opens near the base of the circle of circumoral tentacles. 

 The Holothuroidea generally develop by means of bilaterally 

 symmetrical larva?, which resemble very much the earlier stages 

 of the larvae of the common starfishes. In some groups no 

 larva occurs, but the young develop directly, being retained 

 close to the external surface of the body of the parent, where 

 the}' are protected by tentacle-like structures. The inner por- 

 tion of the body wall is provided with well-developed muscles, 

 arranged in part circularly, passing around the body, and in 

 part as longitudinal bands extending along the ambulacral 

 areas. These muscles assist largely in the locomotion of the 

 animals, especially in those species in which ambulacral feet 

 are wanting ; the tentacles sometimes assist in this function 

 also, but one of their chief purposes is to secure food. At times, 

 especially if seized by other animals or roughly handled, the 

 strong contraction of the muscles of the body wall ejects the 

 viscera from the body, and under favorable conditions these 

 may be formed anew, just as the starfish regenerates its lost 

 arms and their contained viscera. 



The sea cucumbers vary greatly in color, form, and size. In 

 the temperate regions they are generally not brightly colored, 

 being of various shades of brown or buff, or sometimes nearly 

 transparent ; in the tropics there are many species which are a 

 brilliant yellow. They live in the sand and along the shore and 

 at all depths out to the deep sea. In form they range from the 

 simple, cylindrical, wormlike species (Fig. 95) to the elaborate, 



