ZOOLOGY : MARINE INVERTEBRATES. 85 



ing this zone, Purpura lapillus. The shells of Littorina, though 

 not comparatively as thick as those of Purpura, nevertheless are 

 strong and capable of resisting a great force. The barnacles are 

 protected admirably by their corona and the well-fitting valves 

 of the inner shell. The mussel, Mytilus edulis, while perhaps suf- 

 ficiently protected by its shell, seems nevertheless to be inca- 

 pable of sufficiently firm fixation to withstand the force of the 

 waves. Consequently we find it only in sheltered nooks behind 

 rocks, or under the rockweed ; in all of which positions its habit of 

 growing in closely packed beds helps to secure stability of attach- 

 ment for the individual. 



The following two localities, both near together and within 

 easy reach from Boston, are selected to illustrate the littoral fauna 

 of the cliffs, because they furnish also beautiful examples of tide- 

 pools. Other headlands, such as those of Marblehead Neck and 

 Cape Ann on the north, and the Xantasket cliffs on the south, 

 furnish good illustrations of the littoral fauna of the exposed 

 ledges ; but they are not so rich in tide-pools. Nevertheless these 

 ledges deserve close attention; as the degree of exposure, the 

 character of the cliffs, and the force of the waves vary sufficiently 

 to produce at least some variation in the fauna. 



CASTLE ROCKS, NAHANT. 



Route. — From Lynn (see pp. 11-13), barge or bicycle across Nahant 

 neck to Castle rocks, about 5 miles. Barge fare, $.25 round trip. 



A large number of interesting invertebrates always may be found 

 in the tide-pools of Castle rocks, as well as on and under the 

 rockweed which is plentifully exposed at low tide. In this latter 

 habitat the two common hydroids of our rocky coast, Sertularia 

 pumila and Campanularia flexuosa, are always abundant, not in- 

 frequently growing on the rocks under the seawfeed, or in the 

 crevices. The egg-cases of Purpura lapillus and other gastropods 

 are also common under the rockweed, while the animals themselves 

 may be found everywhere, especially where the barnacles upon 

 which they feed are abundant. The shells of the Purpura vary 

 considerably in size, form, thickness, ornamentation and colora- 

 tion ; and in a comparatively short time a large number of speci- 

 mens may be gathered ranging in color on the one hand from pure 

 white to pure orange, purple or dark gray, and on the other from 



