GENERAL DIRECTIONS. 75 



some of the sedentary genera may retire to deep water in 

 certain seasons. Except that one might mention a local- 

 ity where he had collected them, it is next to impossible 

 to direct a collector to a place where the large majority 

 can always be found without failure. 



Among the Hydrozoa the problem of habitat is perhaps 

 more difficult to solve than among the other groups. 

 Most of the free genera are so sporadic in their appear- 

 ance that it is difficult to say where one should go on any 

 fixed date, and not be prepared for failure. The places 

 where these have been taken are so widely spread along 

 our coast that they may be said to occur anywhere along 

 the shore, but for the great majority of nomadic genera 

 there is great uncertainty that at any definite time they 

 can be found in numbers at any one of these places. With 

 the fixed hydroids it is however different, on account of 

 the nature of their habitat. 



The several genera of fixed hydroids prefer as a general 

 thing a rocky bottom just below low-tide mark. Their 

 favorite habitats are rocky cliffs exposed to the sea, or quiet 

 pools left by the retreating tide. They are also fond of 

 the fronds of Laminaria and Fucus, buoys and submerged 

 parts of wharves and landing stages. The bottoms of 

 boats which have been continuously in the water for some 

 time are often covered with these animals. Although the 

 majority are to be found in these and similar places there 

 are a few which are attached to the sand or live in the mud. 



Clava leptostyla, which may be taken as an available 

 type of the so-called Tubularian hydroids can always be 

 found at low tide on the small ledge of rocks near Beverly 

 Bridge. This locality I have repeatedly visited for the 

 purpose of collecting Clava, and have never been disap- 

 pointed in obtaining a large number. It is found attached 

 to the Fucus which hangs from these rocks into the 



