72 The Irish Naturalist. [ March, 



The Mollusca belong to a pretty high type of animal 

 organization, and some naturalists still bring them close up 

 to the Vertebrata in the scheme of classification, which is 

 doubtless raising them too high, and I am sure Sir John 

 Lubbock does not approve of their taking precedence of his 

 wonderful socialistic communities of wasps, bees, and ants. 

 The Mollusca are bilaterally symmetrical, unsegmented 

 animals, i.e., they are without jointed appendages such as the 

 Crustacea and Insects have. The body is covered with a soft, 

 slimy skin which in the Lamellibranchiata and Gastropoda is 

 protected by a shell, the tougher integument of the Cephalo- 

 poda, preserved even in fossils, not requiring this. The 

 Mollusca are mostly aquatic, especially marine animals, 

 though many (Gastropoda) are terrestrial, but even these 

 generally seek damp situations. Breathing is effected in the 

 aquatic kinds by gills, the water entering the mantle-cavity 

 by a small orifice, the edge of which may be drawn out into a 

 tube or siphon, hence called the respiratory siphon — this is 

 obviously useful in the burrowing Molluscs. The second 

 siphon has excretory functions. In the pulmonate Gastropods 

 as the name implies, respiration is carried on by the mantle 

 cavity, acting as a lung. The habits of life of the Mollusca 

 are various ; they may be fixed by their shell-substance to 

 foreign bodies or other shells, as the Oyster, or by silky 

 threads — the byssus — as the Common Mussel, or they may be 

 pelagic i.e., free-swimming in the open sea, as the Cuttle- 

 fishes, or creeping as most Gastropods, or again burrowing 

 in sand or mud, or even boring in rock and wood. Among 

 the latter is the well-known ship-worm Teredo nava/is, which 

 bores into ship's bottoms, wharves, and other submerged 

 wood-work. As to feeding, the Lamellibranchs, sedentary or 

 sluggish as they generally are, must have their food brought 

 to them, it is therefore mostly in the shape of microscopic 

 organisms and particles drawn into the mouth by currents 

 set up by the rapidly moving hair-like cilia of the gills. The 

 Gastropoda, especially those with a respiratory siphon and 

 notch forits passage, are carnivorous, as theWhelk (Buccinum) ; 

 others, as the Periwinkle (Littorina), are sea- weed eaters. 

 The Cuttlefishes are well known as animal feeders, their long 

 tentacles, armed with suckers, enabling them easily to capture 



