CORDYLOPHORA LACUSTRIS. 17 



inhabits our fresh waters. It has been thoroughly in- 



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vestigated by Allman, who has made it the subject of an 

 admirable memoir, which is one of the most important 

 contributions to our knowledge of the structure and 

 physiology of the Athecate Hydroida. 



The polypitcs of Cordylophora, when kept in confine- 

 ment, soon perish, and are soon reproduced. They do not 

 drop off, like those of Tubularia, but are destroyed by a 

 process of absorption. The arms are roughened by thread- 

 cells, which are arranged in regular nodules. The poly- 

 pite, like the Hydra, possesses a remarkable power of 

 elongating and shortening its tentacles. At times they 

 are so much extended as greatly to exceed the entire body 

 in length, and in this state are attenuated into most deli- 

 cate filaments. When contracted they appear corrugated, 

 and comparatively thick. 



The polypite is a singularly beautiful object when the 

 tentacula (some 12 or 14 in number) are all fully elon- 

 gated, floating like gossamer threads through the water, 

 and waving to and fro with its every slightest movement. 



The reproductive buds are produced on the ultimate 

 ramules, at some distance behind the polypite. There are 

 sometimes as many as three on a branch, which are placed 

 alternately, "the more advanced being always nearer to 

 the main stems " ( Allman) . I have counted twelve ova 

 in a single female capsule, but more commonly the num- 

 ber amounts to six or eight. They are developed into 

 ciliated planulse, which escape through the ruptured 

 walls of the sac. 



On reaching the water the embryo remains inactive for a 

 few seconds, undergoing remarkable changes of shape ; the 

 body then acquires a rotatory motion, and it sails off with 

 considerable rapidity. It is elongate-oval in form, somewhat 

 broader at one extremity than the other, opake white in 



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