MULTIVALVES AND BIVALVES. 4! 



There is but one adductor ; the anterior, as in the pectens, having 

 become atrophied owing to the shape of the shell bringing the 

 hinge and two adductors into line. To make up for this, the 

 surviving adductor, the posterior, having more to do, becomes of 

 conspicuous size. 



Above the adductor, on the median line, is the heart, with its 

 single ventricle and two auricles, pulsating in the pericardium as it 

 drives the colourless blood through the system. Below the ligament 

 is the mouth, with its palps, leading down into the gullet and the 

 stomach, and onwards to the intestine. Round the loop of the 

 intestine is the greenish-brown liver, from which the tubes lead into 

 the stomach and carry into it the digestive fluid. 



On the anterior side the gills start from the palps and curve half- 

 way round. The gills are the " beard," which everyone does not 

 appreciate as an article of food, and is often thrown away by the 

 opener. There are two gills, each consisting of two thin plates, flat, 

 but diminishing in thickness towards the outer edge. The blood is 

 brought through the arteries to a venous canal, and thence thfough 

 the renal organ to the gills, whence, after being aerated, it returns 

 by a canal on each side to the two auricles, or rather the one auricle 

 which is in two divisions. Outside the gills, and extending all 

 round, is the mantle, which encloses the body, and is protected by 

 the shell it secretes. 



The oyster has no foot and no siphons, but its general body 

 structure may be taken as representing that of the rest of the class. 

 One family, however, is so unlike the others that some explanation 

 is necessary. The family contains the one genus, Teredo, the ship- 

 worms. The shipworm is a worm in appearance, with a pair of 

 valves protecting one end and a pair of pallets protecting the other. 

 The animal, so far as its principal organs are concerned, is very small, 

 and is contained within the nut-like pair of valves that are without 

 either hinge or ligament. The siphons, into which the gills are pro- 

 longed, occupy eleven-twelfths of the length. These siphons secrete 

 a shelly lining as the animal burrows its way into the wood, and at 

 their point of separation are placed the calcareous pallets which 

 close the tube against intruders. The larger of the siphons takes 

 in the water charged with air and food, and down the smaller tube 

 is ejected the spent water and the woody pulp formed as the animal 

 bores deeper and deeper out of its own danger to the danger of the 

 unsuspecting mariner. 



Our species of Teredo are all easily distinguishable by their 

 pallets, those of bipinnata being, as figured on the coloured plate, of 

 most characteristic structure. They are five times the length of the 

 valves, their blades being composed of about fifty conical joints 

 nested one within another, having feathered edges fringed on each 

 side. The stalk, instead of being short, as in the other species, is 

 long and slender and minutely tuberculated. It may be added that 

 in the plate there are shown a single valve and pallet of each species, 

 with a figure of the tube in the case of No. 167. 



One more note. Brachiopods have shells not unlike those of 

 pelecypods, and are frequently listed with the British mollusca. 

 What a brachiopod may be is rather a puzzle, but it is certainly not 

 a mollusc ; arid it ought therefore to be out of our range. As, 



