MOLLUSCS, OR SHELL-FISH. 231 



of the year; the commoner forms are called sea- slugs or sea- 

 lemons. 



For clearness let us briefly summarise this classification of 

 Gasteropods, or Molluscs in which the foot forms a ventral 

 creeping sole. 



GASTEROPODA. 



1. Zygobranchia, limpet-like forms, generally with 



simple, more or less conical shell. 



2. Azygobranchia, forms like whelk and periwinkle 



with coiled shells. 



3. Opisthobranchia, forms in which the shell is often 



absent and never well developed. 



The Chitonidae with eight shells and eight pairs of gills 

 are often separated from the true Gasteropods. 



We shall take first the limpets as representatives of the 

 Zygobranchia. On the East Coast, at least four of these are 

 common between tide-marks. Commonest of all is Patella 

 vulgata, the limpet of the fishermen, which is too familiar to 

 need description. It is always abundant between tide-marks 

 on rocky coasts, and is often found in little pits or depres- 

 sions of the rock, into which the shell exactly fits. It has 

 been shown by experiment with marked limpets that each 

 limpet has its own particular habitation, to which it retreats 

 as the water begins to ebb. When the rock on which the 

 limpet has settled down is covered again with water, how- 

 ever, the limpet sets out in search of the Algse which form 

 its food. As it travels it forms a broad track, often very 

 distinct where it has crossed sand or muddy rock. Experi- 

 ments on the " homing instinct " of limpets are easily made, 

 and can be carried out at neap tides on days when other 

 shore work is largely stopped. The common limpet is in 

 great demand for bait on most parts of the coast. It seems 

 popular with most fishes, a somewhat curious fact since it is 

 so purely littoral in habit. It is in many ways an interest- 

 ing species, and the student should not fail to watch the 

 way in which the peculiar tongue is used to mow down the 

 small Algae on which it feeds. As it glides over the rocks 

 the long tentacles are moved about in all directions, and 

 show clearly the small eyes at their bases. The position of 

 the eyes should be contrasted with that seen in the garden 



