306 ANIMAL BIOLOGY. [Part II. 



successive additions to the shell during growth, forms, on the 

 dorsal border of the valve nearer the more rounded anterior 

 end, a small blunt eminence, the umbo (urn.). Posterior to this, 

 the exoskeleton is uncalcified for some distance along the dorsal 

 border. This uncalcified part is called the ligament (Ig.) It 

 serves by its elasticity to keep the shell slightly agape. Fig. 89, 

 A., shows the mussel in its normal position partially buried in 

 the mud or sand. A fleshy part of the animal, the foot (/.), 

 protrudes at the anterior end. Posteriorly a short double tube 

 projects from the shell. The dorsal tubular part is smaller and 

 smoother, the ventral part is larger and provided with a number 

 of papillary processes. If the water be slightly turbid, a current 

 will be seen continually setting in at the lower part of the tube 

 (inhalent siphon, in. si.) and out through the upper part (exhalent 

 siphon j ex. si.). 



The mussel may be killed by placing it in cold water and 

 gradually raising the temperature of the water until it is as hot 

 as the hand can bear without discomfort. The shell will be 

 slightly gaping and the foot protruded. On removing the 

 mussel from the water, pulling the valves somewhat further 

 apart, and looking in between them from the ventral aspect, 

 their inner surface will be seen to be lined with a delicate 

 yellowish mantle or pallium. Between the mantle and the 

 fleshy foot are seen on each side two flattened lamellar organs, 

 the gills. These lie in a large chamber (infra-branchial chamber), 

 normally closed below by the meeting across the mid-ventral 

 line of the mantle folds, but open posteriorly by the inhalent 

 siphon, the papillary walls of which are now seen to be a 

 specially differentiated part of the mantle. A probe passed into 

 the exhalent siphon enters a smaller chamber (supra-branchial 

 chamber) partially separated from the infra-branchial chamber by 

 the growing together, or concrescence, of the posterior portion 

 of the inner gill-lamellse behind the foot. The probe (Fig. 90, 

 A., pr.) will, however, emerge from the upper into the lower 

 chamber by a wide slit which separates the inner gill from the 

 foot. Anterior to the foot, between it and a strong band of 

 muscle (anterior adductor) which passes from one valve of the 



