Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/704

Rh 676 MOLLUSCA [CEPHALOPODA. complete internal cartilaginous skeleton than is to be found in some of the lower Vertebrates. There are other instances of cartilaginous endo-skeleton in groups other than the Vertebrata. Thus in some capito-branchiate Chaetopods cartilage forms a skeletal support for the gill-plumes, whilst in the Arachnids (Mygale, Scorpio) and in Limulus a large internal cartilaginous plate the ento-sternite is devel oped as a support for a large series of muscles. Alimentary Trad. The buccal cone of Nautilus is ter minated by a villous margin (buccal membrane) surround ing the pair of beak-like jaws. These are very strong and dense in Nautilus, being calcined. Fossilized beaks of Tetra- branchiata are known under the name of Rhyncholites. In Dibranchs the beaks are horny, but similar in shape to those of Nautilus. They resemble in general those of a parrot, the lower beak being the larger, and overlapping the upper or dorsal beak. The lingual ribbon and odontophoral apparatus has the struc ture which is typical for Glosso- phorous Mollusca. In fig. 107, A is represented a single row of teeth from the lingual ribbon of Nautilus, and in fig. 107, B, C, of other Si- phonopoda. In Nautilus a long and wide crop or dilated oesophagus (cr, fig. 110) passes from the muscular buccal mass, and at the apex of the visceral hump passes into a highly muscular stom ach, resembling the gizzard of a bird (gizz, fig. 110). A nearly straight intestine passes from the muscular stomach to the anus, near which it develops a small caecum. In other Siphonopods the oesophagus is Usually FIG. narrower (fig. 106, oe), and the mus cular stomach more capacious (fig. 106, v), whilst a very important feature in the alimentary tract is formed by the caecum. In all but Nautilus the caecum lies near the stomach, and may be very capacious much larger than the stomach in Loligo vulgaris or elongated into a spiral coil, as in fig. 106, e. The simple . Alimentary canal of Loligo sag ittata (from Ge- genbaur). The buccal mass is omitted, oe, oesophagus ; v,the stomach opened long itudinally ; x, probe passed through the pylorus ; c, commencement of the cae cum ; e, its spiral portion ; i, intestine ; a, ink-bag ; 6, its opening into the rectum. Fio. 107. Lingual dentition of Siphonopoda. A. A single row of lingual teeth of Nautiltu pompilhis (after Keferstein). B. Two rows of lingual teeth of Sepia officiruilis (after Troschel). C. Lingual teeth of Eledone cirrhosa (after Loven). U-shaped flexure of the alimentary tract as seen in fig. 106, and in fig. 110, is the only important one which it exhibits in the Cephalopoda, the Pteropoda (except the Limacinida) agreeing with the Siphonopoda in this sim plicity in consequence of their visceral hump being un twisted. The acini of the large liver of Nautilus are compacted into a solid reddish -brown mass by a firm membrane, as also is the case in the Dibranchiata. The liver has four paired lobes in Nautilus, which open by two bile-ducts into the alimentary canal at the com mencement of the intestine. The bile-ducts unite before entering the intestine. In Dibranchiata the two large lobes of the liver are placed antero-dorsally (beneath the shell in Decapoda), and the bile-ducts open into the caecum. Upon the bile-ducts in Dibranchiata are deve loped yellowish glandular diverticula, which are known as &quot;pancreas,&quot; though neither physiologically nor morpho logically is there any ground for considering either the so- called liver or the so-called pancreas as strictly equivalent to the glands so denominated in the Vertebrata. In Nauti lus the equivalents of the pancreatic diverticula of the Dibranchs can be traced upon the relatively shorter bile- ducts. Salivary Glands are not developed in Nautilus unless a pair of glandular masses lying on the buccal cavity are to be considered as such. In the Dibranchs, on the contrary, one (Sepia, Loligo) or two pairs of large salivary glands are present, an anterior and a posterior (Octopus, Eledone, Onychoteuthis). Each pair of salivary glands has its paired ducts united to form a single duct, which runs forward from the glands and opens into the buccal cavity r.as Fro. 108. Diagram of the nephridial sacs, and the veins which ron through them, in Sepia officinalis (after Vigelius). The nephridial sacs are supposed to have their upper walls removed, v.c, vena cava ; r.d.v.c, right descending branch of the same ; r.s.v.c, left descending branch of the same ; v.b.a., vein from the ink-bag ; v.m, mesenteric vein ; v.g, genital vein ; v.a.d, right abdominal vein ; v.a.s, left abdominal vein ; v.p.d, right pallial vein ; v.p.s, left pallial vein ; c.b, branchial heart ; x, appendage of the same ; c.v, capsule of the branchial heart ; np, external aperture of the right nephridial sac ; y, reno-pericardial orifice placing the left renal sac or nephridium in communi cation with the viscero-pericardial sac, the course of which below the nephri dial sac is indicated by dotted lines ; i/, the similar orifice of the right side ; a.r, glandular renal outgrowths ; w.k, viscero-pericardial sac (dotted outline). near the radula. The anterior pair of glands when present lie in the head near the buccal mass, the posterior pair lie much farther back beneath the liver, at the sides of the oesophagus. It is the posterior pair which alone are pre sent in Sepia and Loligo. The ink-bag is to be considered as an appendage of the rectum. It is not developed in Nautilus, nor in the Pteropoda; in all Dibranchiata (even in the fossil Belemnites) it is present (fig. 106, a ; fig. 103, t), and has been observed to develop as a diverticulum of the rectum, with spirally plaited walls which very early secrete a black pigment. The spiral plaitings of the walls diminish