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member of each pair on one side of the mesial line and one upon the 

 other; or if, as is sometimes the case, these muscles are more consoli- 

 dated, the plane of the longitudinal axis so divides them that the 

 sides are perfectly equivalent. The organs of breathing consist of two 

 coils bearing fringes ; these are situated one on each side of the longi- 

 tudinal axis, and are each complements of the other, being coiled on 

 the sides of the shell, and curved in reverse directions, the one hemo- 

 around to the right, the other to the left. The ovarian masses are, in 

 the higher groups at least, arranged in pairs, two, sometimes three 

 pairs in different groups, but in every case one member of each pair is 

 upon the right and one upon the left. The oviducts, one pair, are 

 similarly disposed. 



The circulatory system consists of a heart, two main systems of 

 ramifying excurrent vessels, and returning lacunes, together with four 

 auxiliary pulsating vesicles. 



The heart is central, but each of the great systems of ramifyino- ex- 

 current and returning vessels, one on each side of the body, is balanced 

 with the other; the two pair of auxiliary pulsating vesicles have one 

 unit of each pair on each side. The mesial position of the heart is to 

 be expected, since that feature is common in the Lamellibranchiata ; 

 the only exceptional cases where the heart is divided and thrown upon 

 either side of the alimentary canal occur in the upper members of 

 that order. Those masses, which are termed livers, and which are in 

 some way accessories of the digestive function, are in two pairs, and 

 situated, one of each pair on either side of the longitudinal axis. 



The main cavity of the abdomen is prolonged into the mantles in 

 two pairs of tubes, the function of which is quite unknown. In these, as 

 in those other features of the organization which we have observed, the 

 disposition is bilateral. The nervous system also exhibits a tendency 

 to form lateral equivalents. The main ganglion is central ; the two 

 lesser ganglia closely connected with it are one right and one left, and 

 the two large nervous centres or loops near the occlusor muscles 

 balance each other. The ramifications of the circulatory and nervous 

 systems are so nearly alike, that they can almost be represented in the 

 same diagram, and in both of them there is the same sort of corre- 

 spondence between those on one side and those on the other as there is 

 between ridges and wrinkles on the right hand and on the left in man. 

 Nearly every main twig, and even the minute ramifications in most 

 cases, are duplicates. 



It would be possible to name other parts of the animal of Brachio- 

 poda where differentiation upon the sides is more or less plainly indi- 

 cated, but since the anatomy of the animals is unfamiliar, and not easy 

 to understand, I will not carry the special description any further ; 

 trusting, however, that enough has already been given to demonstrate 



