CHAPTER IV. 



echinodeemata. 



Animals more or less resembling in structure tlie 

 Sea-urchin. 



Classes. — Cmnoidea, or Stone-lilies. 

 AsTEKOiDEA, Or Star-fishes. 

 EcHiNOiDEA, or Sea-urchins. 

 HoLOTHUKOlDEA, Or Sea-cucuiTibers. 



These are the first animals ia which, we meet with a 

 body-cavity (see p. 40) surrounding, the alimentary 

 canal, and an excretory opening, or anus."^ Lying in 

 this body-cavity there are (i.) the alimentary canal, (ii.) 

 the reproductive organs, male and female, and besides 

 these, structures which have no existence in the 

 Coelenterata, the Vascular System or system of 

 blood-vessels, and also a unique set of structures 

 called the "Water-vasciilar System. This consists 

 in the starfishes of a hollow ring, with little bags or 

 vesicles (bladders), called the Polian vesicles, at- 

 tached to it, and it communicates with a large number 

 of other and much smaller little bags on the outside 



' Some of the higher infusoria- (Pai-amcecutm) have a sepa- 

 rate region for excretion; one of the sea-anemones {Peachia) 

 has been said to reserve one side of the oral aperture for 

 this use; and the Ctenoplwra are exceptional in possessing 

 tvf.o definite excretory apertures. 



