84 



INVEKTEBRATE ANIMALS. 



appear to be present, as a rule ; but tlie mesoderm is usually pro- 

 vitled with scattered nerve-cells. No definite organs of the circula- 

 tion are develojied. Distinct reproductive organs are always present, 

 and true sexual reproduction occurs in all the members of the class. 

 In a great many forms, however, of the Actinozoa, we have com- 

 jiosite organisms or colonics, produced by a process of "continuous" 

 gemmation or fission, the zooids thus oi-iginated remaining attached 

 to one another. In these cases — as in most of the Corals — the 

 separate beings or zoiiids thus produced are termetl " polypes," the 

 term " polyjjite " being restricted to the Hydrozoa. In the simple 

 Aftiiiozoa, however, such as the Sea-anemones, the term " polype " is 



Fig. 47. — Di;i-i'aiiiinatic vprt.iral section of a Si'a- Anernon3. a Month; .5 Stomach; 

 & Borty-ca\-ity; c c Convolnted corfis (" crasperla ") containiii^^ thread-cells, and 

 forn^ing the fi-ee edges of tlie mesentery (jji) ; t f Tentacles; n Reprodnctive organ 

 contained within the mesentery. The ectoderm (e) is indicated by the broad 

 external line, the endi^derm (t') hy the thin line and the space hctwecn that and the 

 ectudeim. 



a]iplied to tlic entire organism, as con.sisting of no more than a 

 single alimentary region. It follows fi'om this, that the entire Imdy, 

 or " actinosoma," of any Actinozoun, m;iy be cimtp(_)sed either of a 

 single polype, or of several such, produced by budding or cleavage, and 

 imited to one anfither Viy a common connecting structure or cwnosarc. 

 Most of the j\iiii-i(>\i«i ai-e permanently fi.xed, like most Corals; 

 some, like the Sca-.-niemimes, pos.sess a limited amount of locomotive 

 power; ;ind idle onler, Ihe < 'ti-nnpliorii, is composed of highly active 

 free-swiniming oiganisnis. Sunie of them are unjirovided with hard 

 sli iictnres nr sujiports of any kind, as the Sea-anemones and Cteno- 

 lilinrn ; but ;i gre.it ni;iny secrete a c;dc;ireous id- lud'ny skeleton or 

 fi-;niii'W(M-k wliicli i.; kno\v;i as the "coral" (d- "coralluni." 



