2 6o ABSORPTION OF INTERNAL PARTS DECOLLATION chap. 



the s])ii'(' lose 

 in which the) 



Fig. 168.— a I' ri- 

 cula Judae 



Lam., showing 

 the disapi)ear- 

 aiice of the 

 partitions of the 

 whorls, which 

 are represented 

 by dotted lines. 

 (After Fischer.) 



the brackish 



their ,s])iral form, and take the shape of the cavity 

 li(\ Amongst the genera in which this singular 

 process takes place are Nerita} Olivella, and Cyiwaea 

 amongst marine forms, and nearly the whole of 

 the Auriculidae ^ (Fig. 168). Conus reduces the in- 

 ternal subdivisions of the spire to extreme thin- 

 ness. It is noticeable that these genera are all 

 of considerable thickness of shell, and it is per- 

 haps the result of the whole energy of the animal 

 being directed to the fdi'iiiation of its external 

 protection that the internal walls of the spire 

 become atrophied and eventually disappear. 



Decollation. — -In certain genera, when the 

 shell becomes achdt, the animal ceases to occupy 

 the upper wliorls, which accordingly die and drop 

 off, the. oriHce at the top having meanwhile been 

 closed by a shelly deposit. Such shells are termed 

 decollated. Tn some land genera decollation is 

 (he rule, c.//. in Cyl iitd rella (Fig. 1G9), Emcdo- 

 diiun, and litoiiina, as well as in many species of 

 water genera, Truncatclla, Ccntlddea, and Quoi/ia. 



Pig. 169.— a, Decollated (adult) 

 form, and B, i)erfect (youny) 

 form of Cj/HndreUa nobiliiu- 

 Ad., Jamaica ; the dotted 

 line shows where decollation 

 takes place. 



Fig. 170. — Development of Coecum : A, 

 showing the gradual formation of 

 septa ; «, ape.x ; ap, aperture ; ss, 

 lirst se])tuni ; s's', second septum. 

 (After de Folin.) B, Adult form of 

 v. ebnrneum Ai\., Panama. x S. 



Stenotjym. {Rumina) decoUata, a common shell in the south of 



1 Mr. B. B. ^Yooaward has recently pointed out (P. Z. S. 1892, !>. 528) a very 

 remarkable method t)!' shell absor[)tiou and growth in J'datca and certain other 

 Neritidae. 



2 The only exception appears to be Fcdipcs, wliile in Cassidula and Scarabus the 

 absorption is partial (Crosse and Fischer, Journ. dc Conch, xxx. p. 177 f). 



