HYDROIDS. 



79 



Fig. 69. — Cordytophora lacustrls. 



found in an old well. These two, Hydra and Cordylophora, are the only hydroids 

 ; known to live in fresh-water. A third, imperfectly known form, allied to Cordylo- 



phora, has been described by Professor Cope, from a 



lake in Oregon. 



In the oceans, hydroids are very abundant, and 



there are at least several hundred species. All of 



them may be arranged in a few groups, most of which 



are represented on our shores. Our first example of 



the marine forms will be Clava leptostyla, a beautiful 



reddish species which occurs on our coast from Long 



Island Sound northward. Its most common habitat 



is at or near low water mark, attached to the rock- 

 weed (Fucus), where it forms colonies consisting of 



numerous individuals attached to a common rhizome 



or branching base. It is about a half of an inch in 



length, and the " head " bears from fifteen to thirty 



irregularly arranged slender tentacles. Beneath the 



tentacles, at the breeding season, the small reproduc- 

 tive buds are arranged in groups as shown in the figure. 



The reproduction is essentially like that of the next 



species. One of the most common forms found in shal- 

 low water (one to twenty fathoms) fromVineyard Sound 



northward, is known as Eudendrium, dispar. It grows in colonies 

 from two to nearly four inches in length, and the parts of the 

 colony which correspond in appearance to the stems and branches 

 of a plant are dark-brown or black. At the tip of each branch 

 and branchlet is a hydra-like animal, or zooid, which is directly 

 connected with every other one in the colony, for the whole 

 colony is strictly compai-able with a much-budded Hydra grown 

 to an equal height, and the general cavity of the body is con- 

 tinuous through all the stems and branches into every zooid. 

 When taken out of the water, however, Eudendrium retains 

 its shape, which Hydra cannot do. This stability or rigidity 

 is due to the existence of a nearly complete coat or covering 

 of horny material, chitin, which is secreted by the animal, and 

 which extends over all the colony, with the exception of the 

 zooids ; they remain unprotected. During the summer months 

 two kinds of Eudendrium may be found along the New Eng- 

 land coast, which are exactly alike in the characters given, but 

 differ in color, one having white zooids, the other yellow. A 

 little careful examination will show that upon the bodies of the 

 white zooids are a series of structures arranged in a circle just 

 beneath the tentacles ; each one of these is in shape like a short 

 string of beads, which are supposed to be male organs, showing 

 that the white colonies are male. The yellow ones are colored 

 by a number of simple bud-like processes which are irregularly 



scattered on the body of the zooids ; they are the female reproductive organs or ovaries. 



In Eudendrium then, the sexes are in different colonies. An egg having been fertil- 



Fig. m. — Clava leptostyla, 

 enlarged ; a, b, a, d, rae- 

 dusse buds. 



