22 THE SEAS 



then a group of walking legs, and finally a series of swimming 

 legs under the tail region. Allied to them are the zoologi- 

 cally mysterious sea spiders or Pycnogonida, resembling 

 the true spiders in little beyond the possession of four pairs 

 of long legs. 



The Mollusca form another great group. The Gastro- 

 poda or univalved shellfish, such as the limpets, periwinkles 

 and snails, the great majority of which are marine (the 

 chief exceptions are snails and slugs), have usually a shell, 

 always of one piece, though the land and sea slugs have either 

 no shell or else one greatly reduced and covered over 

 with skin. Most of them live on the shore or on the sea 

 bottom but a few, without, or with greatly-reduced shells, 

 swim about near the surface, the sea butterflies being the 

 commonest. The bivalve molluscs or Lamellibranchia 

 have a shell composed of two equal, or almost equal, halv3s 

 united by an elastic ligament which forces the two halves 

 open except when they are drawn together by the powerful 

 adductor muscles. They are all either marine or fresh- 

 water animals, well-known examples being the mussel 

 and the oyster. The third great division of the Mollusca 

 includes the octopus, squid and cuttlefish, and is called the 

 Cephalopoda. Its members are all marine and are very 

 highly organized animals with a head having a pair of 

 complex eyes and surrounded with a series of arms possess- 

 ing suckers and hooks. Some, such as the octopus, have 

 no shell ; others, the cuttlefish is an example, have a broad 

 " bone " down the back which is covered with flesh, while 

 a few, like the pearly Nautilus, have a large shell in which 

 they live. 



Resembling the bivalves in the possession of a shell 

 consisting of two valves are the Brachiopoda, usually only 

 found in deep waters in our own seas. The two halves of 

 the shell however, are always dissimilar, while the internal 



