CHAPTER XXX. OBELIA GENICTJLATA. 



1. Form and Occurrence. This organism occurs in 

 two forms, one that of a fixed compound zoophyte, the 

 other that of a free active medusa. The fixed form is 

 called the hydroid stage, the free form the medusoid. The 

 hydroid develops from a fertilised ovum, the medusa is 

 produced by budding from the hydroid. The hydroid is a 

 compound organism consisting of a number of members 

 connected with one another, and produced by gemmation. 

 I propose therefore to call it a gemmarium, a term which is 

 more appropriate than those hitherto employed. It consists 

 of small upright stems attached by their bases to the surface 

 of sea-weeds, and is very common on our shores on the 

 fronds of Fucus and Laminaria. Each stem is about an 

 inch in length, and the members of the compound are 

 " zooids " attached alternately to opposite sides of the 

 stem (Fig. 201). 



2. Structure of Zooid. Each zooid has a structure 

 similar to that of a Hydra. It consists of a body wall 

 composed of ectoderm, mesogloea, and endoderm, enclosing 

 a simple cavity. At the distal end of the zooid is an 

 aperture, the mouth, while at the attached end the body 

 passes into a cylindrical stalk or peduncle, which is con- 

 nected with the stem, and the stem itself is tubular. The 

 part immediately below the mouth is conical and is called 

 the hypostome ; this is about one third of the length of the 

 zooid, and is separated from the main part of the body by 

 a circle of tentacles, which are narrow outgrowths of the 

 body-wall, longer than the hypostome. Each tentacle con- 

 sists of ectoderm externally and endoderm internally, but 

 it contains no cavity, the endoderm consisting of a single 

 row of vacuolated cells. Between the ectoderm and endo- 

 derm there is a layer of mesogloea. The stem to which 



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