38 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 Pleurobrachia rhododadyla is the commonest example^ 



. It is like a ball of transparent jelly, mov- 



^^"^^^X^ ing through the sea by means of eight 



/ yll\»Vv ^°^® °^ minute paddles, and throwing 



f M 11 W 1 1 ^^^ from a sac on each side of the body 



\l Wl Jla . Jra 1/ two long ciliated tentacles. 



\w^^fshW/ These beautiful animals derive their 



/ ^ 1 L ^\ class name from the vertical rows of 



comb-like paddles (ctenophores) situ- 



^ /-'■^^~\ ^-ted on eight meridional bands of mus- 



L^ Sr\\^,^ cles which serve as locomotive organs, 



/ ^ISLilL M^rN They have a true digestive canal passing 



V^Ifll^^SJ/'^ through the body. Like other animals 



V^ 1 W7^ ^^ *^^ ^^^^^ ^^ ^^^^ nourishes the body 



^~€^_^^ by means of canals passing off from the 



Fig. 36a.— View, of thestomach, and called gastro- vascular ca- 



gastro-vascular canals -i 

 of a Pleurobrachia, naiS. 



traSitearms'hIveTJet Reviewing our steps up the scale of 

 sfdSrthe miutHpTn^ li^^j ^"^ scc that wc have come to radi- 

 f?f m the miuth end.- ^^* a«i»J«!?.s witli a dishuct mouth sur- 

 After Gegeubaur. rouiided hy tentacUs contaming thread- 

 cells, opermig into a gastric cavity winch is used both for 

 digestion and respiratton ; that they have a distinct nervous 

 system, often 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 Ctenophora are called Cmlenie- 

 rata in allusion to their rudimentary digestive cavity. 



Classes op Ccblenterata. 



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

 brella-shaped animals with a nervous system. By drosod (Hydra). 



3. A digestive sac held in place by partitions; 



polyp often forming coral .Actinozoa (Actinians). 



3. Body round or oval, swimming by series of 



comb like paddles Ctenophora. 



