THE ANIMAL KINGDOM 



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provided with a fringe of usually six tentacles around its open, 

 free oral end. Its body wall consists of two layers of cells: the 

 outer one, ectoderm, and the inner one, endoderm. Between 

 these two layers is a thin noncellular, supporting tissue, the 

 mesoglea. The endoderm lines the gastrovascular cavity and the 

 tentacles. Thus each tentacle is a double-walled tube, whose 

 cavity, reduced in size, is a continuation of the gastrovascular 

 cavity (Fig. 219A). The medusa type is illustrated in a simple 

 form by Gonionemus, the convex side of whose bell-shaped body, 

 the exumbrella, corresponds to the attached blind end of the 



Fig 219. — Diagrams for comparing the polyp type, A, with the medusa 

 type, B. 1, ectoderm; 2, endoderm; b, bud; g, gastrovascular cavity; m, mouth; 

 o, ovary; t, testis; v, velum. Mesoglea is shown in solid black. 



polyp; while the open end of a tube, the manubrium, leading 

 from the concave surface of the bell, corresponds to the oral 

 end of the polyp (Fig. 219B). In jellyfishes, which are large 

 medusoids, the mesoglea is a thick jellylike layer containing cells 

 that have immigrated from the ectoderm and endoderm. Both 

 polyp and medusa types are radially symmetrical. The term 

 " Coelenterata " (hollo w-intestined) refers to the presence of the 

 gastrovascular cavity in these animals. 



The ectoderm of Hydra is composed of epitheliomuscular cells, 

 interstitial cells, and nerve cells. The epitheliomuscular cell of 

 the ectoderm has the form of a short column whose inner end is 

 drawn out at right angles to the main axis of the cell and contains 



