A FRESH \VATI;K JlvI.I.V I'lSII. 79 



ganic matter through roots, they put forth heads with mouths, 

 stomachs, and, generally, with grasping organs, by whose 

 agency they are nourished with animal food ; and, instead of 

 bearing flowers ripening their seeds "within theinselvcs, the 

 medusoid's correspondent organs are liberated, live and grow 

 (sometimes to a large size) independent, free-swimmers, dis- 

 tributing their eggs and young at great distances ; in turn, to 

 grow up into uriv hydioids, the parents of other medusee. 



The following is a brief history of Microhydra ryderi . 

 I found it in 1885 living as a messmate among colonies of 

 certain polyzoa that were like itself of a fixed habit of growth, 

 upon stones, gathered in Tacony Creek, a mill stream flowing 

 into the Delaware River ; and, later, in the Penns^'lvania 

 Canal, at Flat Rock Dam, on the Schuylkill River; in both 

 situations far above tide water. I have called it a messmate, 

 not only to describe its assoeiatio/i , but the purpose of it, as 

 will appear. 



When seen as a larva, this hydroid is an inert, cylindrical, 

 worm-like body, about one-fiftieth of an inch in length, with 

 a breadth of about one-fifth its own length. No organs at first 

 exist, by which it may move or feed itself, but in a week or 

 two, one end becomes attached to the surface upon which it 

 has lain and the other gradually develops a head or capitulum 

 covered with 30 to 40, so-called, lasso or thread cells, resem- 

 bling those of the common Hydra, but without a trace of ten- 

 tacles. The mouth is hidden at the extremity of this head. 



Imagine, then, its disabilities as a self feeder. Fixed in 

 ]iositii)n, with a power of motion that can barely be detected 

 even when highly magnified, it reminds the observer of the 

 "dog under the rich man's table," dependent almost entirely 

 upon falling "crumbs" or small organisms that are drawn 

 down by the whirlpool vortices created by the relatively pow- 

 erful polyzoa standing around them. This single form is often 

 made bi- or tri-capitulate by branching near the point of 

 support. 



