24 Bulletin Wisconsin Natural History Society [Vol. 11, Nos. 1-2 
(2) they may be active and hard-shelled, or (3) small and ubi- 
quitous; (4) most of them are very hardy and can stand consid- 
erable exposure. 
The ability to cling to a hard surface is of prime importance on 
a rock beach and in the most exposed situations those organisms 
survive which are best able to hold on (figs. 13, 14). The shapes 
and methods of attachment of Fucus, Mytilus, Acmaea, and B al- 
arms are admirably suited to withstand the force of waves and 
these genera succumb in the order given (Balanus being the first 
to cover a clean exposed rock surface and the last to leave it). 
The snails, Littorina and Purpura , are not far behind these 
hardy pioneers; and the attached coelenterates, like Metridium, 
Fig. 19. Balanus Attached to Littorina litorea , Acmaea testudinalis, and 
Mytilus edulis. 
Fig. 20. Limulus polyphemus bearing Crepridula fornicata and C. plana. 
Sagartia, and hydroids, are also well suited to life on the rocks. 
The distribution of some animals is apparently limited only by 
the presence of something hard for attachment; Mytilus is abun- 
dant on the rocks everywhere — even on the mud flats; Littorina 
is common on the firmer portions of sand beaches. Often one 
sessile animal is fixed to another (fig. 19). 
Representative examples of the active hard-shelled animals are 
the crabs, Cancer borealis , C. irroratus, Carcinides maenas, and 
Pagurus pubescens. All these species are able to withstand con- 
siderable beating by the waves, yet they can escape exposure to 
the sun by migrating downward with the receding tide. 
