528 ZOOLOGY. 



(Fig. 553), now rarely met with, but which once existed in 

 great numbers in the seas of various geological epochs. The 

 holothuria are remarkable for the disposition of their respira- 



Fig. 554 Holothuria; Sea Cucumber, 



tory apparatus, composed of membranous tubes ramified like 

 a tree, and receiving water into the interior through the 

 intermedium of a cloaca or anus. 



CLASS OF THE ACALEPH^. 



618. The acalephse are soft animals, of a gelatinous 

 consistence, always floating in the sea, and formed essentially 

 for swimming. Their organization is very simple ; the skin 

 is not distinct from the subjacent parts, and their internal 

 organs are reduced to a cavity or stomach, communicating 

 with the exterior by a single opening, and giving rise to 

 canals extending into the different parts of the body, and 

 there ramifying, so as to give a resemblance to a vascular 

 system. 



The family of this class which is best known is that of the 

 medusae, amongst which are the rhizostomes (rhizostomatidse), 

 which abound on the coast, and which are remarkable for the 

 singular disposition of the digestive apparatus, the stomach 

 communicating externally by a great number of small canals, 

 terminated by pores at the free extremity of the tentacles. 

 In this class are included the heroes (of the class ciliograda) 



