METAZOA COELENTERA. 101 



between a reduced and a primitive form. While resem- 

 bling each other in a general way, close examination 

 reveals here, as in the great majority of cases, the evolu- 

 tionary stages through which the hydroid has passed. 



Cladonema (No. 121) is another Tubularian which 

 illustrates suppressed development. The model repre- 

 sents the hydroid of natural size (a) and enlarged (b) ; c 

 and d are the young and the adult medusae. When old, 

 the medusa tends to lose its umbrella and takes on other 

 reduced characters. 



Tubularia larynx E. & S. (No. 122), is one of our 

 common hydroids and is a good example of suppressed 

 development. The zoons rise from basal stolons which 

 form a network and grow to the height of four or five 

 inches. The network and ascending tubes are protected 

 by a chitinous sheath, but this does not cover the main 

 body of the zoon, often called the hydranth. Each 

 hydranth has a double row of tentacles. Below these on 

 the reproductive zoons are the grape-like clusters of 

 medusae which never become detached. 



The scarlet colored masses of Clava (No. 123) are 

 abundant on the New England coast. The larval hydra 

 becomes attached and forms a colony like the parent. 

 The model represents the nutritive zoons with many ten- 

 tacles, and the reproductive zoons are in clusters beneath ; 

 as these are medusae which never become free, we have 

 here another illustration of suppressed development. 



HYDROPHORA. HYDROIDEA. 



The fresh-water Microhydra ryderi Potts, 1 is described 

 here since it produces free-swimming medusae 2 , which, 



1 Amer. Nat., XIX, 1885, p. 1232. Quart. Journ. Micr. Sci. 

 XXX, 1890, p. 507. 



2 Potts, Amer. Nat., XXXI, 1897, p. 1032. 



