242 On the Form, Growth, and Construction of Shells. 



It maybe seen at a glance that many shells are bivalve, like 

 the oyster and cockle (see Coloured Plate, Fig. 12, Isocardia) ; 

 while a few, called u Chitons " (see 

 Coloured Plate, Fig. 14, Chiton squa- 

 mosus, and Woodcut (1) of Chiton 

 piceus) are multivalve ; hut the great 

 majority are univalve (see same Plate, 

 Figs. 1 — 10), and sometimes tent- 



Fig. l, CMton piceus, shaped (Fig. 6, Patella), or tubular 

 (side view). ( Fig> l3 } Aspergillum; Fig. 8, Denta- 



lium ; Fig. 9, Vermetus ; Fig. 10, Siliquaria), but for the most 

 part spiral (Figs. 2, 3, 4, 7, 9, and 10), though exhibiting an 

 endless diversity in their proportions, as well as in their 

 sculpturing and colour. 



All insects, crabs, and other articulate animals are sym- 

 metrical, having the organs in pairs, i.e., the right side like the 

 left. In corals and star-hshes the bilaterality is usually dis- 

 guised by a radiate arrangement of the parts. But in snails 

 the symmetry of the eyes, tentacles, and other organs of the 

 head, is lost in the body of the creature. Instead of a double 

 heart, and two series of gills, these organs are single, and 

 placed on one side. When on the left side, the growth is from 

 left to right, to provide space ; but in shells which are sym- 

 metrical, like the pearly nautilus, the keyhole limpet, and 

 the Ampullar ia, the gills are developed symmetrically on each 

 side, or nearly so. 



The tendency to grow in a spiral form is very characteristic 

 of the class Mollusca. Some writers have accounted for it in 

 a very matter-of-fact way. " Molluscous animals are long and 

 worm-like ; therefore Nature has coiled them up, that their 

 tails may not be an incumbrance to thena." 



It is easily ascertained that the snail has a small spiral 

 shell when it first quits the egg, and the young whelk may be 

 examined while still a prisoner in the same capsule with its 

 brothers and sisters. 



The convenience of the arrangement is obvious, and that 

 may be sufficient for us at present, but the time is coining 

 when naturalists will desire to look more closely into these 

 things. 



How happens it that the embryo-snail coiling itself up 

 closely in its narrow cell, almost always takes a direction from 

 left to right, following the course of the sun, and forming a 

 dextral spiral, or right-handed shell, like an ordinary screw ? 

 Such a course is not absolutely necessary, neither is it acci- 

 dental. A few whelks and garden-snails — perhaps one in ten 

 thousand — arc lefb-hmided ; and certain kinds of whelks and 

 land-snails (see Coloured Plate, Fig. 2) are as frequently reversed 



