ZOOLOGT : MARINE INVERTEBRATES 79 



be kept alive easily for several days, provided it has a suflficient 

 amount of water. It seems to require periodical exposure, for we 

 seldom find it growing much below low-water marlc. The same 

 species inhabits the piles of all our bridges across tide-streams and 

 estuaries, often occurring where the water is very muddy. It 

 lives also on exposed shores, growing on and under the rockweed 

 and in the fissures of the rock, wlierever protected from the force 

 of the waves. 



A little lower down the piles are covered thickly with mussels, 

 Mytilus edulis, which in turn frequently are covered by the Cam- 

 panularia Jlexuosa. This grows on the mussels in thick bunches, 

 from which it is more easily obtained than from the timbers. 



Almost at low-water mark, and exposed only at the lowest tides, 

 we find another campanularian, Obelia commissuralis, which is 

 occasionally abundant, although one usually must search for it. 

 It is distinguished easily from Campanularia Jlexuosa by the 

 longer, more slender, and more profusely branched form of the 

 hydrozoarium. Obelia gelatinosa, a somewhat coarser species 

 but similar in its general form, also may be found in this position. 

 In the same zone we find the beautiful tubularian, Pai-ypha crocea, 

 which is often very abundant. This is the largest and perhaps 

 most beautiful of our common hydroids ; and is easily recog- 

 nized by the large heads, the two circles of tentacles, and the 

 brilliant pink color. Not infrequently the heads are broken off, 

 the stems remaining like bunches of stubble. Nevertheless they 

 retain their vitality, and probably have in company with other 

 tubularians the power of regeneration. This species grows very 

 rapidly, and will become attached to objects submerged only a 

 short time. A dory which had been for two weeks in the tide-way 

 near the draw of Beverly bridge in June, 1897, had its bottom 

 covered completely with this beautiful bydroid, most of the indi- 

 viduals apparently in an adult stage. 



Below the zone of tubularians, the piles are covered with sea- 

 anemones, Metridium marginatum. These grow so thickly that 

 in places there is hardly a space between them. They vary greatly 

 in color, but the majority are reddish-brown. At the lowest tides 

 many of thera are exposed, and then hang down limp and unsightly. 

 Among them, as well as among the hydroids, nudibranchs are not 

 uncommon. Dendronotus arborescens, with two rows of large ar- 

 borescently-branched gills on its back, is the most abundant. This 



