DEVELOPMENT OF FLSSURELLA, CYPRAEA, ETC. 26 I 



Europe, has been noticed to hang 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 graduully 

 dis.appears and the marginal slit becomes an apical hole, A, B, C, higlily magni- 

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



Special Points in the Growth of Certain Genera. — Tn the 



A'oung of Oacciim. 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 tlie last w^liorl. As 

 growth advances, shelly 

 matter is deposited on hotli 

 mary;ins, which results in 

 the slit becoming a IidIc 

 and the spire a men^ cal- 

 losity, until at List tliey 

 ap])ear to coalesce in tlie 

 i\\)vx of the adult shell (Fig. 

 171) Tlie sinoular forma- Fig. 172.— Three stages in the growth of Cypraea 

 ^' ' o 17-,,. exanthema L. (From specimens taken at 



tioiis of Magilvs and Rhizo- Panama.) 



chilus have already l)een 



described (pp. 75, 70). Cyj^raca, in tlie young stage, is a thin 



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



