20 



MARINE AND FISHERIES 



1-2 EDWARD VII., A. 1902 



manner of spawning, development, change of form and of habits in the young, rate of 

 growth, &c., &c., which demand time, patience, trained observation, and inventive 

 experimentation to elucidate. 



THE EXTERNAL FEATURES OF THE CLAM {Mya arenavia.) Plates I and ii, Figs. 1, 2. 



Size. — Mya arenaria, the common clam, is a mollusk about four inches in length, 

 two and a half inches in depth, and one and a half inches in breadth. Specimens may 

 be found side by side varying considerably from the dimensions here given They have 

 been reported six to eight inches in length on the one hand, and of course they occur of 

 all sizes down to the verge of invisibility on the other. What is generally regarded as' 

 a mark of the adult animal is its ability to deposit eggs or sperm, but the acquisition of 

 this power does not mean the arrest of further growth. 



Shell. — One of the first features to be observed in the clam is that the animal is 

 supplied with a strong, hard shell into which the soft living parts may be withdrawn. 

 The shell is composed of two valves which occupy the same position with reference to the 

 inclosed animal as the cover of a book does to its printed pages. The valves are convex 

 externally, concave internally, and are held together at one margin by a sort of hinge, 

 while at the opposite margin they are capable of being brought together or separated at 

 will. The hinge margin marks the dorsal surface or back of the animal, and the open 

 margin is the ventral surface. It will be noticed that the two halves of the shell are 

 not exactly alike in size, shape or markings, and that one valve doubles over the margin 

 of the other at the hinge. This is the right valve, the other, or smaller one being the 

 left. If a clam is placed before the observer with its hinge uppermost, the larger valve 

 to the right, and the smaller to the left, it will then be in its natural position for loco- 

 motion in the direction in which he is looking. The end turned away from him is its 

 anterior end, and that turned towards him is its posterior end. It is lengthened antero- 

 posteriorly, compressed laterally, while dorso-ventrally it measures less than its length, 

 but more than its breadth. It consequently possesses three axes of different 

 lengths — a longitudinal, a vertical and a transverse. The greatest breadth is 

 just below the hinge, towards the ends and below it gradually narrows. At the 

 ends the two valves do not fit close against each other, but are left ' gaping ' — 

 hence the British name of 'Gaper,' or *Sand Gaper.' Each valve, viewed 

 from the side, is oblong or somewhat oval in outline, with a series of concentric 

 markings parallel with the margin below but narrowing to smaller and smaller 

 dimensions as they approach the hinge. The more or less angular prominences 

 near the hinge, where the concentric lines are smallest, are called the umbones or beaks. 

 The right umbo is the larger. Starting from one of the beaks, the concentric lines in- 

 dicate the different sizes of the shell at different periods, and were caused by temporary 

 suspensions in the desposition of shell matter, followed by renewed activity when the 

 increased growth of the animal required an enlargement of the shell. They must not 

 be considered annual rings of growth, since the greater number of them originate during 

 the first year of the animal's life. The shell is an exoskeleton, secreted by, supporting, 

 and giving protection to the underlying parts. The greater part of its material is cal- 

 cium carbonate (limestone), which produces an effervescence, or an evolution of bubbles, 

 when hydrochloric acid is dropped upon it. On its outside may be found a thin, brown, 

 horny, epidermal layer (periostracum), more or less worn off except in the creases and at 

 the margins where it may also be found to continue on to certain of the more exposed 

 soft parts of the animal. Under this, or coming to the surface where the epidermis is 

 absent, is the thick, prismatic, porcellaneous layer, composed of polygonal calcareous 

 prisms deposited side by side at right angles to the surface. Underneath this and only 

 to be seen from the inside by taking off one of the valves, is the third layer of the shell, 

 the nacreous or pearly layer, composed of numerous superposed films of calcareous mat- 

 ter. When a clam is taken unawares and before it has time to contract, or when it is 

 left quiet for some time in a large glass of fresh sea-water, there may be other parts 

 exposed, such as the siphons, the mantle and the foot. 



