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AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



Fig. 34. MUSSEL, ? . A, Section passing through pedal ganglia, p.g, 

 showing stomach, s, surrounded by digestive gland, d.g; labial palps, 

 l.p, and foot, /, projecting into branchial chamber, laterally bounded 

 by mantle, m. B, Section passing through heart. Above is seen the 

 ventricle, vn, traversed by rectum, r, with auricle, aw, on each side. 

 Below pericardium are seen ureters, u, with vena cava, v.c, between 

 them ; and kidneys, Jc, with cerebro- visceral connectives (black) be- 

 tween them. Projecting into branchial cavity is visceral mass showing 

 intestine, i, cut through in six places, and foot, /; also inner and outer 

 gill-plates, i.g and o.g, on each side ; ax, gill axis on each side traversed 

 by afferent branchial vessel (black) ; the outer gill-plates are full of 

 Iarva3, and # is placed in each of the outer supra-branchial passages, 

 the smaller inner passages are seen above the inner gill-plates. The 

 branches of the ovary, which fill up the greater part of the visceral 

 mass, are omitted. C, Section passing through visceral ganglia, v.g. 

 The rectum, r, is seen above the posterior adductor, p. add. The inner 

 gill-plates have united together behind the visceral mass ; a # is placed 

 in each outer supra-branchial passage, and in the median passage formed 

 by union of two inner ones. In a section still further back these three 

 channels would be found to have coalesced to form the cloacal chamber. 

 Gill axes as in B. D, Small part of longitudinal section through outer 

 gill (enlarged), showing the two gill-plates formed by union of flattened 

 gill-filaments, f, supported by chitinous rods (dark dots). Between 

 the plates are seen two water-tubes, ic.t, separated by an interlamellar 

 junction, i.l.j, in which is a blood-vessel, b.v. The plates are per- 

 forated by numerous small apertures, |, f , leading into the water- 

 tubes. E, Outer parts of six gill-filaments, further enlarged, showing 

 supporting rods. 



