154 



CHAPTER XXV. 



SIMILAR ANIMALS WITH DIFPERENT SHELLS.— TOOTH-LIKE CJiCUM, AND 

 THE SPIRAL TURRITELLA. — CECUM SPIRAL IN EMBRYO.— HABITS OP THE 

 TURRITELLA. 



TURKITELLIDJE. 



I. C^CUM. — It is curious to find, crawling under a minute 

 tubular shell, resembling a BentaUwn, or tooth-shell, a mol- 

 lusc so much like the Twrritella, that it is placed by the 

 authors of the ^ History^ in the same family, and might 

 almost have been placed in the same genus, but that the as- 

 sociation of two such very different shells would have been 

 too much for the nerves of us poor conchologists. It is a 

 remarkable instance, tending to show how httle reliance is to 

 be placed on the mere character of the shell, in any syste- 

 matic arrangement of Mollusca. So long as the tiny crea- 

 ture can withdraw his head and foot into the shelter of his 

 glassy tent, it seems to matter very little whether the upper 



