474 



CHARACTERS OF INVERTEBRATE ANIMALS 



A. 



put adjoining compartments into communication. The stomach, 



which is bounded by the thickened free edges of the septa, is 



of necessity continuous with the 

 various compartments, except when 

 digestion is going on, at which time 

 the edges of the septa are brought 

 close together, a digestive juice being 

 poured out from them upon the 

 food. These points will be made 

 clear by examination of the accom- 

 panying diagram. It may further be 

 noted that the sides of the gullet are 

 furrowed by two grooves placed 

 opposite to each, and this organ caa 

 be collapsed in such a way as to con- 

 vert these grooves into narrow tubes, 

 these being lined by long cilia, which 

 work in such a way that a current of 

 water runs inwards in one tube, and 

 outwards in the other. 



The minute structure corresponds 

 in many ways to that of Hydra, 

 there being ectoderm, endoderm, and 

 a supporting lamella between them. 

 Nettling-cells are abundantly present,, 

 but are more complex in pattern 

 than those possessed by Hydra. 

 There is further a greater amount of 



specialization, as seen especially in the muscular and nervous 



systems. 



The Anthozoa include two orders: i. Six-rayed Sea-Flowers 



(Hexactinia); and 2. Eight-rayed Sea-Flowers (Octactinia). 



Fig. 290. Sea-Anemone 



A, External view of a Sea- Anemone (Anemonia). 

 B, Diagrammatic cross-section; MES., mesenteries: 

 only one of the two food -grooves is lettered, 

 c, Diagrammatic longitudinal section, showing a 

 mesentery (MES.), with mesenteric filaments (MES. 

 KILT.) on the right, and one of the spaces between 

 two mesenteries on the left ; EX., EX'., excretory 

 pores: the arrows indicate the course of currents 

 into and out of the gullet along the food-grooves. 



Order i. SIX-RAYED SEA-FLOWERS (Hexactinia) 



These are distinguished by the fact that the tentacles are 

 simple, while they and the mesenteries are usually arranged in 

 multiples of six, and though this is often difficult to easily make 

 out in the adult, it is clearly shown during the development. The 

 Sea- Anemone just described is a good type of the order. Among 



