DEVELOPMENT OF FISSURELLA, CYPRAEA, ETC. 



261 



Europe, has been noticed to bang its upper whorls violently 

 against some hard substance, as if to get rid of them. 



Fig. 171. — Four stages in the growth of Fissurella, showing how the spire gradually 

 disappears and the marginal slit becomes an apical hole, A, B, C, highly magni- 

 fied, D, natural size. (After Boutan.) 



Special Points in the Growth of Certain Genera. — In 



the young of Ooecum the apex is at first spiral, but as growth 

 proceeds and the long tube begins to form, a septum is produced 

 at the base of the apex, which soon drops off. Soon afterwards, 

 a second septum forms a little farther down, and a second piece 

 drops off, leaving the shell in the normal cylindrical form of the 

 adult (Fig. 170). The development of Fissurella is of extreme 

 interest. In an early stage it possesses a spiral shell, with a slit 

 on the margin of the outer 

 lip of the last whorl. As 

 growth advances, shelly 

 matter is deposited on both 

 margins, which results in 

 the slit becoming a hole 

 and the spire a mere cal- 

 losity, until at last they 

 appear to coalesce in the 

 apex of the adult shell 

 (Fig. 171). The singular 

 formations of Magilus and 

 Rhizochilus have already 

 been described (pp. 75, 76). Cypraea^ in the young stage, is a 

 thin spiral shell with a conspicuous apex. As growth proceeds, 



Fig. 172.— Three stages in the growth of Cypraea 

 exanthema L. (From specimens taken at 

 Panama.) 



