88 



FIRST LESSONS IN ZOOLOGY. 



Besides the class of jelly-fishes or hydroids, and that of 

 polyps, there is a third class, called Ctenophora, of which 

 our Pleurobmchia rhododactyla is the commonest example. 

 It is like a ball of transparent jelly 5 mov- 

 ing through the sea hy means of eight 

 rows of minute paddles, and throwing 

 out from a sac on each side of the body 

 two long ciliated tentacles. 



These beautiful animals derive their 

 class name from the vertical rows of 

 comb-like paddles (ctenophores) situ- 

 ated on eight meridional bands of mus- 

 cles which serve as locomotive organs. 

 They have a true digestive canal passing 

 through the body. Like other animals 

 of the class the food nourishes the body 

 by means of canals passing off from the 

 FIG. 36. View of thestomach, and called gastro-vascular ca- 



gastro-vascular canals -, 

 of a Pleurobmchia, nalS. 



Reviewing our steps up the scale of 

 we see that we have come to radi- 

 atumalf, with a distinct mouth sur- 

 After Gegenbaur. rounded by tentacles containing thread- 

 cells, opening into a body-cavil y which is used both for 

 digestion and respiration ; that tlici/ have, a distinct nervous 

 system, of ten eyes and ears; and that in the jelly-fishes there 

 is a series of changes of form called a metamorphosis. 



All polyps, jelly-fishes, and CtenopJiora arc called Ccelcnte- 

 rata in allusion, to their rudimentary duji'slice cavity. 



CLASSES OF CCELENTERATA. 



1. Hydra-like; adults free-Swimming, bell- or um- 



brella-shaped animals with a nervous system. ffi/<lror.oa (Hydra). 



2. A digestive sac beld in place by partitions; 



polyp often forming coral Actinozoa (Actinians). 



3. Body round or oval, swimming by series of 



comb like paddles Ctenophora. 



