78 



STAR-FISHES, SEA-URCHINS, ETC. 



obstructions from the surface of the test, where this 

 is necessary, can be easily seen. 



SEA-CUCUMBERS. ~ 



The sea-cucumbers, or holothurians, constitute an 

 interesting group of animals whose members live 

 both upon the rocks and buried in the sand or mud. 

 Although so different in gen- 

 eral appearance, they repre- 

 sent only extreme modifica- 

 tions of the structure seen in 

 the star-fish or the sea-urchin. 

 Take for example that singu- 

 larly attractive creature the 

 Synapta, whose elongated 

 leech-like body can be se- 

 cured from the mud-flats by 

 the aid of a garden-trowel, and 

 examine it. The transparent 

 cylindrical form, permitting 

 the yellow intestinal canal to 

 be clearly visible in the in- 

 terior, shows at first little to 

 connect it with either star-fish or sea-urchin, but 

 soon you will perceive five well-defined bands 

 traversing the length of the body from one ex- 

 tremity to the other. These are indeed the am- 

 bulacra, although in this instance the tubes are 

 closed and, so far as locomotion is concerned, func- 

 tionless. In its fundamental structure, therefore, 

 the sea-cucumber is only a greatly elongated sea- 

 urchin, being pushed out axially, as it were, to its 



Synapta. 



