THE STUDY OF SHORE ANIMALS. 27 



the best plan is to take fair-sized specimens of the two 

 forms and compare them point by point. You will notice 

 at once that, just as in the case of the limpets, there is 

 much general resemblance. In all the essentials of structure 

 the two are similar, but there is nevertheless a well-marked 

 difference. Study the two until you can say precisely 

 wherein the difference lies (the shape of the dorsal shield, 

 or carapace, and of the numerous legs will be found 

 especially important), and then return to the young speci- 

 mens. If your analysis has been careful you will find that 

 the difficulty has vanished, and that you can now sort your 

 specimens without fear of error. 



There are many other common animals which can be 

 similarly employed as a means of strengthening the percep- 

 tion of form in animals, and such introductory training will 

 be found of much value afterwards. It will also serve to 

 familiarise you with the haunts and habits of the common 

 types, a point of much importance. As, however, your 

 acquaintance with the rocks and their inhabitants increases, 

 you will find the need of a classification of animals a 

 method of pigeon-holing your too numerous facts. We shall 

 therefore consider next an outline classification. 



The first point to notice is that the fauna of the rocks is 

 so abundant and so varied that, among Invertebrate or back- 

 boneless animals at least, there are few great groups which 

 are not numerously represented. The sea, the fruitful 

 mother of all things, retains representatives of most of her 

 children within herself. In spite, therefore, of the fact that 

 our classification is professedly based on marine forms only, 

 we shall find few important blanks in it. 



Lowest of all, and including forms with which we shall 

 not concern ourselves much here, are the PROTOZOA, the 

 primitive unicellular organisms, resembling those from which 

 all others have originated. Consisting as they do of single 

 cells, or of colonies of cells in which the units are not 

 dependent upon one another, it will be readily understood 

 that the Protozoa are mostly minute, often excessively so. 

 Many forms, however, make shells of lime or flint, and may 

 by their abundance give rise to considerable deposits. Such 

 Protozoa helped to form the chalk of the South of England, 

 and are forming the oozes (Globig&rina ooze and Radiolarian 



