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AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



are concentrically disposed, which mark successive additions made 

 during the life of the animal. The outside is olive green in colour, 

 owing to the periostracum, a thin horny membrane, which also 

 forms a flexible margin to the shell. Within the periostracum 

 there is a prismatic layer, and the shell is lined by a pearly layer, 

 marked by a number of muscular impressions. Near the anterior 

 end there is an oval anterior adductor impression, and near the 

 posterior end a similar but larger posterior adductor impression. 

 The two are connected by a pallial line running parallel to the 



KrS 



Fig. 32. GENERAL DISSECTION OF UNIO (from Glaus, after Grobben). 

 V.S and H.S, anterior and posterior adductors; M.S, labial palp; F, 

 foot; Mt, mantle; K, gills; C.g, cerebro-pleural ganglion; P.g, pedal 

 ganglion; M.g, visceral ganglion; 0, mouth; M, stomach; L, digestive 

 gland; Kr.S, crystalline style; D, intestine; Af, anus; G, genital 

 gland; A, exhalent aperture; E, inhalent aperture; N, nephridium; 

 Vh, auricle; Hk, ventricle ; V. A, anterior aorta ; H. A, posterior aorta : 

 P, organ of Keber. 



ventral edge of the shell. Immediately behind the anterior 

 adductor impression is a much smaller anterior retractor impression, 

 behind which again is a small protractor impression. Just in front 

 and above the posterior adductor impression, there is a small 

 posterior retractor impression. From the various places above 

 mentioned lines of shifting converge towards the umbo, indicating 

 the successive positions occupied by the impressions during earlier 

 stages of growth. 



