ACALEPn,! AND ECHINODERMS. 



163 



work of vessels, in which the blood is exposed to the oxygen- 

 ating influence of the water, whilst the rhizostome floats like 

 a gigantic animalcule through the sea. The aliments gain ad- 

 mission to the stomach 

 only throueh these ab- 



J o 



sorbent tubes, which re- 

 mind us of a type of 

 structure so common in 

 plants ; in the Medusa 

 aurita the mouth is large 

 and patent, and can be 

 closed by a sphincter 

 muscle ; the stomach is 

 divided by septse ; in 

 these cavities fishes are 

 sometimes found, in dif- 

 ferent states of digestion. 

 The ciliograde tribe, 

 as in the Beroe pileus, 

 have a digestive tube, 

 passing straight through 

 the body : from the walls Fi - * 73. Rhizostoma Cuvieri. 



' 



of which numerous vessels take their origin, to traverse the 

 structure of this most elegant acalephe, the marvels of whose 

 organization can only be understood after patient observation 

 with the microscope. 



[ 316. The ECHITSTODEEMS afford a striking illustration of 

 the law of progressive development, in the structure of their 

 skeleton, and internal organs. In the Asterias the mouth is 

 surrounded by tubular tentacula, and protected by fasciculi of 

 spines ; the short esophagus leads into a capacious stomach, 

 occupying the central disc, provided with a mucous lining, 

 and covered by a muscular layer ; from the stomach branches 

 proceed into each ray ; around these canals a number of csecal 

 processes cluster, regarded as rudimentary glands : in Ophiura 

 and Euryale the csecrJ processes are absent. In Comatuhi, 

 which connects the sea-stars with the urchins, the stomach 

 occupies the central disc, and leads into a long intestine, which 

 makes two turns around that organ. The mouth forms a larse 



*~* 



opening at one side of the under surface, and the intestine 

 terminates in a prominent aperture, at the opposite side. Ill 



M 2 



