MOLLUSCA — CEPH ALOPODS 255 



The mouth is at the free extremity of the head region and is 

 surrounded by slender tentacles, sixty in the male, ninety in the 

 female ; these are situated on the edges of a series of fleshy 

 lobes which represent the anterior part of the foot of other 

 mollusks. Each tentacle consists essentially of two parts, — 

 a slender cirrus and an elongate, tubular sheath into w^hich the 

 cirrus can be retracted for protection. 



The tentacles serve the double function of prehension and ad- 

 hesion, being used for seizing food and for attachment to surfaces. 

 They are provided with longitudinal, circular, oblique, and trans- 

 verse muscles so that they are well equipped for a variety of 

 muscular services. There are no true suckers as in the squid, 

 but a kind of sucker-like efficiency is imparted to the tentacle 

 by the contraction of certain of its muscles. Thus there are 

 formed definite suctorial ridges on the lower and inner surfaces 

 of the tentacles, and Nautilus can hold to the surface of attach- 

 ment w4th considerable tenacity. The lower portions of the 

 tentacles on the inner or dorsal side of the head are fused into 

 a thick muscular lobe, the hood, which serves like the operculum 

 of gastropods to close the aperture of the shell w^hen the body is 

 withdrawn into the living chamber. On the ventral side of the 

 head region is the fuimel or hyponome (Fig. 112, f.) ; this looks 

 like a thick, muscular leaf with free, roUed-in edges. The posi- 

 tion of this funnel is indicated on the exterior of the shell by the 

 hyponomic sinus, a dowm-bending of the lines of growth on 

 the ventral (outside) portion of the shell. 



The short, rounded body is enveloped by the mantle, which is 

 prolonged posteriorly into the fleshy, hollow tube of the siphon, 

 extending backward within the siphuncle of the shell through 

 all the otherwise empty chambers. Anteriorly the mantle is 

 produced into a free flap surrounding the head and attached to 

 the shell near the edge. When the large mantle cavity is opened 

 there are seen the two pairs of gills, the two pairs of osphradia, 

 and the sac-like body wall containing the viscera. Opening 

 from this sac into the mantle cavity are the apertures for the 

 discharge of the waste and reproductive products. 



