124: ZOOLOGY. 



CHAPTER V. 



CCELENTERATA AND PROTOZOA. 



SUB-KINGDOM VI. Ccslenterata. 

 CLASS 1. Actinozoa. 



178. The Actinozoa (Gr. aktis, a ray ; zoon, an animal) 

 is the highest class of the Ccelenterata. It includes the 

 sea-anemones, the corals, the sea-pens, and some of the 

 jelly-fishes. These animals are exclusively marine. 



The Actinozoa are defined as ccelenterate animals, in 

 which the 'mouth leads to an alimentary canal, which, in- 

 stead of being continued through the body, and terminating 

 at the exterior, communicates by a wide aperture with the 

 general cavity of the body. This canal is separated from 

 the body walls by a space, which is divided into compart- 

 ments by a series of vertical partitions called mesenteries 

 (Gr. mesos, the middle ; enter on, intestine). To the sur- 

 faces of these mesenteries the ovaries and sperm cells are 

 attached. 



179. The body consists of two separate layers an outer 

 and an inner. The outer is termed the ectoderm (Gr. ectos, 

 without ; derma, skin), and the inner the endoderm (Gr. 

 endon', within; derma, skin). The interior of the body 

 is filled with fluid, in which a circulation is promoted by 

 the cilia that line the inner walls of the endoderm. The 

 only digestive organ is a stomach, in the form of a tube, 

 which can only communicate with the exterior through 

 the mouth. ]STo heart or circulatory organs have been 

 discovered in any member of this class. In one group 

 only ( ' Ctenopliora) is there any trace of a nervous system. 

 The sexes are often united in the same individual. Some 

 are simple, others are compound. Colonies are formed 

 by budding, or by self-division of a single individual, as 



