TETRABRANCHIATA. 



831 



continuous with the shell-wall, are known as the " sutures," and the 

 form of these necessarily varies with that of the septa themselves. 

 In one great division of the Tetrabranchiates (viz., the Nautiloidea) 

 the septa are simply curved or slightly lobed, and the " sutures " 

 are more or less completely plain. In the other great division of 

 the order (viz., the Ammonoided) the septa are folded and complex, 

 and the " sutures " are angulated, zigzag, lobed or foliaceous (fig. 

 742). The form of the "sutures" cannot be determined from the 

 exterior, so long as the shell is preserved ; but in fossil forms, in 

 which the shell itself is commonly more or less extensively removed, 

 the form of the sutures can be readily studied, and affords charac- 

 ters of high morphological value. 



The " siphuncle " in the Tetrabranchiate Cephalopods has the 

 form of a tube connected in front with the hinder end of the 

 body, and continued backwards throughout 

 the entire series of air-chambers. The pre- 

 cise function of the siphuncle is uncertain, 

 but it probably serves to maintain the vitality 

 of the shell. In the Pearly Nautilus the 

 siphuncle is a membranous tube, the walls 

 of which are to some extent strengthened 

 by the deposition of calcareous granules. 

 In some fossil Tetrabranchiates these calca- 

 reous deposits are so extensively developed 

 that the wall of the siphuncle may become 

 completely calcified. The proper sheath of 

 the siphuncle, formed in this way, must, 

 however, be carefully distinguished from cer- 

 tain other calcareous envelopes which may 

 come to more or less completely enclose 

 the siphuncular tube. Thus, in the Pearly 

 Nautilus each septum is prolonged backwards 

 at the point where it is perforated by the 

 siphuncle as a short shelly tube — " neck " 

 or "funnel" — which encloses the anterior 

 portion of each segment of the siphuncle. r F }s- 743-— Partial section 



r. 1 ° of the shell of A tuna at?iri, 



In some 1 etrabranchiates these septal showing the " septal necks " 

 necks" do not merely form short collars JS tt SfS^SSS ^ 

 round the siphuncle, but each is continued 



backwards from the septum in which it originates to the septum 

 next behind (fig. 743, si) ; so that the proper siphuncle becomes 

 enclosed in a complete secondary investment. In the majority 

 of the Nautiloids the septal necks are directed, as in the Pearly 

 Nautilus, backwards from each septum, and such forms are said to 

 be "retrosiphonate." On the other hand, the majority of the 



