84 



INVERTEBBATE ANIMALS. 



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

 vided with scattered nerve-cells. No deiinite organs of the circula- 

 tion are developed. 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- 

 posite organisms or colonies, produced by a process of " continuous " 

 gemmation or fission, the zooids thus originated remaining attached 

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

 separate beings or zooids thus produced are termed " polypes," the 

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

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



Fig. 47. — Diagrammatic vertical section of a Sea- Anemone, a Mouth; s Stomach; 

 6 Body-cavity ; c c Convoluted cords ("craspeda") containing thread-cells, and 

 forming the free edges of the mesentery (m) ; ( ( Tentacles ; o Reproductive organ 

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

 external line, the endoderm (e") by the thin line and the space between that and the 

 ectoderm. 



applied to the entire organism, as consisting of no more than a 

 single alimentary region. It follows from this, that the entire body, 

 or "actinosoma," of any Actinozooii, may be composed either of a 

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

 united to one another by a common connecting structui'e or cccnosarc. 

 Most of the Actinozoa are permanently fixed, like most Corals; 

 some, like the Sea-anemonea, possess a limited amount of locomotive 

 power ; and one order, the Ctenophora, is composed of highly active 

 free-swimming organisms. Some of them are unprovided with hard 

 structures or s\ipports of any kind, as the Sea-anemones and Cteno- 

 phora ; but a great many secrete a calcareous or horny skeleton or 

 framework which is known as the " coral " or " coiullum." 



