THE STUDY OF SHORE ANIMALS. 



31 



The last group of Invertebrate animals, or those without 

 a backbone, is the MOLLUSOA, including Bivalves, like mussels, 

 scallops, cockles, etc. ; 

 Gasteropods, like peri- 

 winkle and whelk; cut- 

 tles, like squid and 

 octopus. The Mollusca 

 have soft, unsegmented 

 bodies usually covered 

 by a shell secreted by 

 a fold of skin called 

 the mantle, but many 

 shore Molluscs have no 

 shell. They usually 



breathe by gills Or by FIG. ll.-Common scallo 



the mantle, and have A bivalve Mollusc. 



a very characteristic 



muscular protrusion called the foot, which is usually the 



organ of locomotion, and is well seen in the garden snail, 



where it forms the creeping surface. 



The Vertebrates of the shore include the FISHES, easily 

 recognised without special description, and the TUNICATES, or 

 sea squirts, a strange set of animals much modified and not 

 easy to recognise. Those on the shore are of two kinds 

 the simple forms which are little shapeless sacs found under 

 stones and overhanging rocks, and the compound forms 

 which consist of little stars within a sheet of jelly-like 

 substance which spreads over rocks and stones. The simple 

 forms have two openings at the upper end, from which on 

 an alarm they eject jets of water. They are enveloped in a 

 usually tough tunic, which can be torn off, and reveals the 

 soft body beneath. Of the details of structure something 

 will be said later. 



The outline classification of shore animals just given may 

 be summed up in the following table : 



Invertebrata. 



Animals which at DO time of life have a backbone or any 

 similar structure down the back. Gill-slits, or openings 

 between the mouth-cavity and the exterior, present in fishes 



