THE COELENTERATES AND CTENOPHORES 89 



The term polyp, which is appHed to the hydroid individual, is 

 derived from the Latin name of the cuttlefish (Polypus) because 

 of a superficial resemblance between the two animals. 



Cell Layers. — The body wall is composed of an external layer 

 of cells called the ectoderm and an internal layer lining the 

 gastrovascular cavity called the entoderm. Between these lies 

 a non-cellular, gelatinous substance termed the mesoglea. The 

 extent and importance of this mesoglea in the various coelen- 

 terates varies extremely. In hydroids it is commonly a very 

 delicate, inconspicuous layer, while in jellyfishes it has become the 

 most conspicuous as well as the most bulky part of the whole 

 organism. Structures having their origin in either ectoderm or 

 entoderm tend to pass into the mesoglea, thus rendering it more 

 highly complicated and causing it to partake of the nature of a 

 definite mesoderm. The extreme of this tendency is found in the 

 Ctenophora, a group which is frequently included as a class of the 

 Coelenterata but here recognized as an independent phylum. 



Tissues and Organs. — In degree of differentiation, the medu- 

 soid represents much the higher type. Structures frequently 

 either wanting or of low organization in the hydroid individuals 

 are well represented in the medusae even of the same species. 

 Muscle occurs as a partial differentiation of epithelial cells in 

 hydroids of the Hydrozoa and Scyphozoa but sheets and bundles 

 of muscle tissue are recognizable in the medusae. Specialized 

 sensory organs are almost exclusively associated with the more 

 distinctly centralized nervous system of the medusoid forms. 

 In these, organs of equilibrium appear in extremely diverse 

 stages of development, representing conditions varying from 

 simple, exposed, modified tentacles or sensory clubs to the highly 

 complicated statocysts with accessory protective vesicles (Fig. 

 39). The reproductive organs typify the lowest degree of 

 specialization, for no ducts or other accessory sexual organs of any 

 sort accompany the gonads. 



Modifications of the Digestive System. — In its most primitive 

 condition, the coelenteric or gastrovascular cavity of coelenterates 

 is a simple bag with a single opening, the mouth. This is the 

 condition in the polyp of the Hydrozoa. In the medusae, there 

 are frequently diverticula from the central chamber of the 

 digestive system which provide greater space for the digestion 

 of food and serve for the delivery of digested food material to the 

 more distant parts of the body. In many instances, these diver- 



