XII 



PHYLUM MOLLUSCA 



771 



Of existing Dibranchiata, Spirula alone has a shell (Fig. 701) 

 comparable to that of Nautilus. The shell of Spirula is of spiral 

 form, the turns 

 of the spiral, 

 however, not 

 being in con- 

 tact. Intern- 

 ally it is divided 

 into chambers 

 by a series of 

 septa, and these 

 are perforated 

 by a siphuncle. 

 But the initial 

 chamber (proto- 

 conch}, instead 

 of being, like 

 the initial 

 chamber in 



Nautilus simi- FIG- 701. Shell of Spirula. A, outside view ; B, showing last 



1 ,1 chamber and position of siphuncle ; C, in section, showing the 



lar tO trie Otners septa and the course of the siphuncle ; D, sh3ll broken to show the 



f-li ,-vn Ji inllor. convexity of the inner side of the septa; E, portion of a septal 



tnougn smaller, ueck (After Cooke .) 



is dilated into 



a spherical shape, constricted off from the succeeding chamber, 



and has passing through it a tube the prosiphon not continuous 



with the siphuncle. Again, as will be 

 seen by comparing Figs. 684 and 702, 

 the relation of the soft parts to the shell 

 is the reverse of what obtains in Nautilus, 

 the shell of Spirula curving backwards 

 (endogastric curvature), that of Nautilus 

 forwards (exogastric curvature). Moreover 

 the shell of Spirula is an internal structure, 

 being almost completely covered by the. 

 mantle. 



The shell of the extinct Ammonites (Fig. 

 703), which are usually referred to the 

 Tetrabranchiata, resembles that of the 

 Nautilus in many respects, being a cham- 

 bered spiral shell with a large terminal 

 chamber, and with a siphuncle. The chief 

 external difference is in the form of the 

 sutures, or lines of union of the edges of 



m b2srtBK Bl *'SSS52s the se P ta with the side wal1 of the she11 ; 



sucker ; /. funnel ; si, * 2 . these are more or less complexly lobed, 



projecting portions of the . -, f , . .. . -./ . ./ -,> 



>n L the internal part of instead oi being entire as in JNautilus. nm 



in one important respect the shell nf nn 



