COELENTERATA 67 



General Consideration of the Coelenterata 



Distribution. — The majority of the Coelenterates are found in 

 salt water, where they are extremely numerous. The common 

 fresh water hydra and a species of fresh water medusa are the only 

 inland types. 



Anatomy. — All Coelenterates have a body wall consisting of two 

 layers of cells (ectoderm and entoderm) and an undifferentiated 

 mesoglea. Digestion and circulation take place in a single coe- 

 lenteron. Muscle fibers aid in the process of locomotion. Loco- 

 motion is active in the jelly fishes like Gonionemus, which ejects 

 jets of water, but less rapid in the larger jelly fishes, which execute 

 undulatory movements. Hydra loops and somersaults. 



Physiology. — Digestion is largely extracellular in some coe- 

 lenterates, the enzymes being discharged into the gastrovascular 

 cavity. In others it is an intracellular process. The endoderm 

 cells responsible for digestion and absorption are ameboid in 

 character, in some cases apparently fusing to form a syncytium. 

 In many Coelenterates, cilia or flagella bring about a slow circulation 

 of the liquid, which may be termed gastrovascular circulation. 

 Tryptic ferments found in some Coelenterates probably furnish an 

 acid secretion as in protozoa. Digestion of animals with a chitinous 

 covering is effected in the anemones by mesenterial jilmnents which 

 penetrate to all parts of the body and there digest and absorb food 

 matters. There is little or no evidence of free existence of pro- 

 teolytic enzymes in the gastral cavity. Glandular cells empty their 

 secretion into the gastric cavity, where it becomes liquid and 

 evidently has a part in what may be termed predigestion of food. 

 Products of this early digestion may be, and doubtless are, carried 

 to the most distant parts of the body by a sort of circulation, some- 

 times termed gastrovascular. This group appears to present a 

 sort of transition between purely intracellular digestion as it appears 

 among the protozoa and purely extracellular digestion found in 

 higher animals. 



Respiration and excretion are performed through the body wall 

 and solid wastes are extruded through the mouth. 



Nervous System. — In the Coelenterates we find that specialized 

 ectodermal cells receive and transmit stimuli to internally situated 

 contractile cells. The nerve net consists of a diffuse network be- 

 tween the receptor and effector cells. Parker ^ calls the units of 



^Parker, G.H. 1919. Elementary Nervous System. J. B. Lippincott Co., N. Y. 



