86 GUIDE TO LOCALITIES. 



variegated and mottled to regularly striped and banded. Nearly 

 smooth varieties occur side by side with those coarsely ribbed, 

 and occasionally an individual with strong varices may be found. 

 Variations in elevation of spire and outline of peristome also 

 occur. The two small species of Littorina, L. rudis and L. pal- 

 liata, are always abundant on the rocks or seaweed, both show- 

 ing considerable variation in form and color. Both species are 

 vegetarians living upon the seaweed (Verrill). The first of these 

 is of special interest from being viviparous (Verrill). Around the 

 stems of the rockweed, on the more exposed ledges, where the 

 cool clear water bathes them the [greater part of the time, our 

 common furry bryozoan Alcyonidium hispidum usually may be 

 found. When the polyps are withdrawn the bryozoarium is not 

 very attractive ; but when the specimen is placed in a jar of clear 

 cool water the pale purple polyps soon expand and the colony 

 becomes an object of great beauty. 



In the fissures left by worn-out dikes, as well as under the over- 

 hanging portions of the ledges, various sponges of the genus Haly- 

 chondria (?) may be found. The most abundant of these have 

 a light greenish tint, and occur also in the tide-pools. With them 

 lives a bright red species. 



Numerous small hollows or diminutive tide-pools may be found 

 on these ledges, and they furnish the best opportunity for observ- 

 ing the barnacles in the operation of feeding. In order to see 

 this, one must get down close to the pool, in such a position as 

 not to obstruct the sunlight. Unless the water has become too 

 warm, the little creatures almost always will be found active. In 

 these small pools, various isopods and amphipods are common. 



The larger tide-pools, always fringed with rockweed and lined 

 with Corallina, Chondrus crispus, Ulva, and the long-streamers of 

 the smaller laminarians, are the ideal collecting grounds for the 

 student of marine invertebrates. A few interesting ones occur at 

 Castle rocks, but there is probably no place on the Atlantic coast 

 where tide-pools are so abundant and so rich in life as at East 

 point, Nahant. The larger tide-pools of Castle rocks always are 

 well stocked with Modiola modiolus, star-fish, and sponges ; and in 

 addition to these a number of the more delicate invertebrates oc- 

 cur. Among the rock masses, where protected from the force of 

 the waves, the common sea-anemone, Metridium marginatum, lives 

 in abundance. When the tide is out these animals can be observed 



