Metazoa Obeli a. 221 



upright stems spring from the rooted portion, each stem 

 having smaller branches. There may be few or many up- 

 light stems, and the root- formation or stolon may show a 

 corresponding variability in extent. Each branch is termi- 

 nated by a zooid, and the entire structure is a colony, [he 

 persons of which are thus intimately connected with each 

 other. 



The stolon is a branching and anastomosing network of 

 delicate tubes, the cavities of which are perfectly continuous 

 with each other and with the upright stems which spring from 

 them. The tubes of the stolon are covered externally by a 

 horny, laminated cuticle, known as the perisarc, enclosing 

 a cellular axis or coenosarc. The outer layer of the coeno- 

 sarc shows no very clear division into cells, though nuclei 

 are scattered at intervals through it, pointing to the fact 

 that the layer is of cellular origin. The cavity of the tube 

 is lined by a layer of cells bearing cilia. To the outer or 

 protective layer the name of ectoderm is given ; to the 

 inner or digestive layer, that of endoderm. These two 

 terms must on no account be confounded with the terms 

 ecto- and endo-sarc used in describing the differentiated 

 layers of protoplasm in the Protozoa (p. 56). The endoderm 

 and ectoderm are layers of cells ; the ectosarc and endosarc 

 are layers of protoplasm of one cell. The structure of the 

 upright branches is the same as that of the stolon, save that 

 the central cavity becomes larger and more distinct as the 

 ends of the various branches are reached. 



It will be advantageous at this point to digress and 

 examine another related Hydrozoon known as Hydractinia. 

 A common species (ff. echinatd] is found covering the shells 

 of many hermit-crabs. Like Obelia it consists of a rooted 

 branched stolon and upright stems. But the stems are short, 

 and each is terminated by a single zooid. Four distinct 

 types of zooid can be identified. One known as the ali- 

 mentary zooid consists of an upright stem or body, con- 

 tinuous with the stolon at its fixed extremity, and terminated 



