Report on the Invertebrata of Massachusetts, comprising the Mollusca, Crustacea, Annelida, and Radiata . e, arranged somewhat like ruffles behind thefoot, enveloping the abdominal mass. Fourth, the siphons, whichare the openings for the passage of excrement, and for the admis-sion of water to the branchiae ; sometimes they are very long, andtheir tips are usually fringed. There are also two thin, elongatedstrips on each side, attached to the mouth and passing back overthe sides of the foot, which are supposed to be the principal or-gans for the detection and selection of food, and are called p
Report on the Invertebrata of Massachusetts, comprising the Mollusca, Crustacea, Annelida, and Radiata . e, arranged somewhat like ruffles behind thefoot, enveloping the abdominal mass. Fourth, the siphons, whichare the openings for the passage of excrement, and for the admis-sion of water to the branchiae ; sometimes they are very long, andtheir tips are usually fringed. There are also two thin, elongatedstrips on each side, attached to the mouth and passing back overthe sides of the foot, which are supposed to be the principal or-gans for the detection and selection of food, and are called palpi. This representation ofthe animal of MACHiERAnitida may serve to ex-emplify the parts above-mentioned. The valves are kept closed by strong muscles, which pass fromone valve to the other. When these are relaxed, the valves arethrown open by the elastic nature of the cartilage or ligament atthe hinge. The uniting substance is called cartilage when it isplaced between the edges of the valves, and ligament when it issituated externally. The interior of the shell exhibits the indentations and lines. MOLLUSCA. 25 where the muscles and mantle or pallium are attached ; and arecalled the muscular and palleal impressions. The latter usuallyturns inward towards the centre of the shell, and then outwards,forming a notch or sinus, more or less deep, at one end of theshell ; this is always the hinder end. There is sometimes only one muscidar impression, but gen-erally there are two, in each valve. These serve as permanentmarks by which to divide the conchs into two groups, the Uni-musculosa and Bimusculosa. The following synopsis may enable one to form a near approxi-mation to the genus of any of our bivalve shells. It has beendrawn up without any reference to a natural arrangement, andmerely to aid the student, by grouping the shells in an artificialmanner, to come more easily at their true place and name. I. Shell transverse or rounded, with hoo muscular impressions. S
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