MOLLUSCS, VERMES AND HYDROZOA 



animal matter; the whorls turning about a central columella or pillar, with 

 the outer or body whorl always the larger. The apex is the first growth 

 of the shell and subsequent development is marked by lines of growth in- 

 dicating a former position of the aperture. When the suture or depression 

 between the whorl, from the apex to the aperture, forms a right hand spiral, 

 the shell is right-handed or dextral, and when turned to the left is left- 

 handed or sinistral. The shell grows by the addition of new layers on the 

 lip or outer edge, but the inner portion of the mantle also forms calcareous 

 matter to increase the thickness of the shell with age. The outer coating 

 is the harder, but the action of the acids in the water often erodes it, 

 whereby the shell may lose some of the whorls. When the erosion has 

 penetrated the interior lining or nacre and the acidulated water comes into 

 contact with the animal, death results. 



Reproduction is by eggs, and some of the genera are oviparous, 

 depositing the spawn to hatch, while others ire ovoviviparous and hatch 

 the young in the oviduct of the female. The oviparous snails deposit 

 translucent gelatinous masses in which the yolks of the eggs are visible, 

 which enlarge and gradually assume the form of tiny snails with trans- 

 parent shells and escape from their gelatinous covers to the plant. The 

 ovoviviparous snails bury themselves in the mud and silt and bring forth 

 a number of perfect young, which, though very small, exactly resemble 

 their parents. Some species carry the young with them, inside the shell, 

 until they have grown to considerable size. 



Bivalves. Most of the Bivalves have double-hinged valves or shells, 

 a tonguehke foot, sometimes provided with a byssus or tuft of threads 

 with which to attach themselves. The body of the animal consists of a 

 mantle of two lobes, leaflike gills, one or more siphons or orifices, a heart, 

 stomach, liver, intestines, reproductive organs and muscles. Fig. T41. The 

 valves increase in size by the addition of concentric rings to the outer 

 edges and in thickness by deposits of nacre on the inner side; the umbone 

 or nucleus being the original shell of the young mussel. The valves are ' 

 united by a hinge which varies in structure with the different species, and 

 are kept closed by adductor muscles connecting them at each end. The 

 interlocking projections of the hinge are known as teeth, and according to 

 their location are cardinal when placed under the beak, anterior and poste- 

 rior lateral when placed before or behind the umbones. 



A set of muscles constitute the anterior and posterior retractors and 

 protractors which control the foot. Mussels have no distinct head, the 

 mantle covering the entire animal and is attached to the valves by a mem- 

 brane. Under it, on the posterior end, are the gill plates, partly separated 

 by the foot and the viscera, and forming a large cavity. In front of the gills 



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