The _Teredinidæ_, or Ship-worms, are also borers, like the Pholads, but do not perforate rocks. They are principally wood-borers; the large _Kuphus arenaria_, which is an exception, living buried in the sand. The ship-worm has a long worm-like body, from 6 to 12 inches in length, which is more or less enclosed in a thin shelly tube or sheath. The true bivalved shell is at the thicker end, and protects the mouth, labial palps, the liver, and other internal organs. At the opposite, or more slender, end of the animal, the mantle is produced into two small tubes, one of which conveys the water to the gills, whilst through the other the water is expelled, charged with the woody pulp excavated by the foot. At the end there is a pair of pallets, or paddles as they are sometimes termed, which are probably used as a means of defence, in closing the shelly tube after the contraction of the siphons. [Illustration: Fig. 37. Ship-worm (_Teredo norvegica_). Case 202. _a_, animal, removed from its shelly tube: _p_, _p_, pallets; _s_, exhalant siphon; _s′_, inhalant siphon. _b_, _c_, different aspects of the shell. ] These animals are most destructive to ships, piers, etc.; and wood, which is not protected by metal, when once attacked, is soon riddled through and through. They work either with or across the grain, and although the holes may be all but touching, they seldom appear to run into one another. [Illustration: Fig. 38. Watering-pot Shell (_Brechites vaginifer_). Case 204. _a_, bivalve shell of the very young animal. ] The “Watering-pot shell” (_Brechites_), of the family _Clavagellidæ_, is a very remarkable structure, and unlike the shell of an ordinary bivalved mollusc. On looking carefully, however, near the perforated end (the rose), two small valves will be seen imbedded in the surface. They are found with the rose downwards buried in mud or sand at low water on the shores of the Indian and Pacific Oceans. SEPTIBRANCHIA. (Fig. 25, D.) [Sidenote: Case 204.] The members of this order differ from other Lamellibranchs in having the gill-plates represented by a muscular _septum_. They are provided with two respiratory siphons and two adductor muscles, and the edges of the mantle-lobes are connected at three points. The families _Poromyidæ_, and _Cuspidariidæ_, constitute this order. The species are all small, without colour-markings, are world-wide in their distribution and occur at all depths. Class V.—CEPHALOPODA.[9] [Illustration: Fig. 39. A, the upper, B, the lower beak of _Architeuthis monachus_; one-third natural size. ] [Sidenote: Cases 205–208.]