MOLLUSCA. 109 



I 



maries united by a transverse commissure ; spermiducts comparatively 

 short. Nervous System. Post-oesophageal ganglion comparatively small. 

 Development. Hatched as a somewhat shrimp-like Mysis larva, the last 

 five pairs of thoracic limbs of which are provided with slender exopodites, 

 afterwards lost, and which has no abdominal appendages. 



The Sea Crayfish or Norway Lobster (Nephrops) closely resembles the 

 common lobster, but the antenna possesses a large squame, and the gill- 

 plume of the first podobranch (that of mxpd. 2) is rudimentary or absent. 

 The number of gills on each side in the three forms is, therefore, as follows, 

 writing p, a, and pi for podo-, arthro-, and pleurobranchs, and omitting 

 rudiments : 



Astacus, 6p + 11 a + I pi = 18. 



Homarus, Qp + 10a + 4 pi = 20. 



Nephrops, 5p + lOot + 4?>Z = 19. 



CHAPTER VII MOLLUSCA. 

 11. ANODONTA and UNIO (Fresh Water Mussels). 



THESE are bivalve molluscs found in considerable numbers in the- 

 mud at the bottom of many ponds, streams, and canals, in this 

 and other countries. Mature specimens may be as much as five 

 or six inches long. 



MORPHOLOGY AND PHYSIOLOGY. 



1. External Characters. The bilaterally symmetrical body is 

 unsegmented, flattened from side to side, and entirely enclosed 

 in a calcareous bivalve shell, cuticular in nature, and consisting 

 of a right and a left piece (valve). These two valves are hinged 

 together dorsally by the ligament, an uncalcified band of horny 

 elastic fibres, which in the closed shell are on the stretch. In 

 dead specimens there is nothing to keep the valves closed, and 

 they consequently " gape," owing to the elasticity of the ligament, 

 but in the living animal their edges can be closely approximated 

 by means of two strong adductor muscles, running across from 

 valve to valve. Each valve is oval with a rounded anterior and 

 a more pointed posterior end, a strongly curved ventral margin, 

 and a straight dorsal margin near the front end of which is the 

 umbo or oldest part of the shell. Around the umbo lines of growth 



