ACALEPHJE 



ECHLN'ODERMS. 



163 



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

 ; influence of the water, whilst the rhizostome floats like 

 -rantic animalcule through the sea. The aliments gain ad- 



-n to the stomach 

 only through these ab- 

 sorbent tubes, which re- 

 mind us of a type of 

 structure so common in 

 plants ; in the Medusa 

 ta 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 y 

 have a digestive tube, 

 Mg straight through 

 the body; from the walls Fi S- 173. Rhizostoma Cuvieri. 



of which numerous vessels take their origin, to traverse the 

 structure of this most elegant acalephe, the marvels of whose 

 iiization can only be understood after patient observation 

 with the microscope. 



[ 316. The Eciiiis'ODEHMS 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, 



pying 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 Eiiri/ale the csecal processes are absent. In Comatula, 



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 large 



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



.1 Kites in a prominent aperture, at the opposite side. In 



M 2 



