1042 CEPHALOPODA. 



tions to this rule being Nautilus, Gonatus, and the Cirrhoteuthidce, the 

 latter of which have no radula. In front of the radular prominence 

 is the so-called " tongue," a fleshy projection covered by a somewhat 

 thick papillated cuticle ; it corresponds to the subradular organ of 

 other molluscs. 



The salivary glands, of which 2 pairs are present in many Cephalo- 

 poda, pour their secretion into the buccal cavity. 



The true stomach is a more or less globular or elongated pouch, 

 with fairly thick muscular walls, and is situated at the summit of the 

 visceral mass ; its 2 orifices, the cardiac and the pyloric, are anterior. 



The liver is formed by 2 symmetrical glands, which are separate 

 from one another during development, but are generally partially 

 fused together in the adult. 



With the exception of Nautilus, Cirrhoteuthis, Polypus arcticus, and 

 P. piscatorum, all the Cephalopoda, including the fossil Belemnites, 

 have an ink-sac consisting of a highly developed rectal caacum developed 

 early in embroynic life from the dorsal wall of the intestine and open- 

 ing into the extreme part of the rectum. This sac is made up of a 

 deeper part, or gland proper, the cavity of which is septate, and a 

 reservoir, into which the glandular part opens by a very small orifice : 

 the reservoir specially well developed in the Decapoda. The Cephalo- 

 poda are able at will to expel the secretion contained in the reser- 

 voir of this anal gland through the funnel, and thus conceal themselves 

 by producing a dense cloud in the water. An oxydizing diastase, 

 called tyrasinase, is concerned in the production of the secretion, the 

 latter being known as melanin, or commonly as sepia, the painters' 

 colour, which is chiefly obtained from Sepia officinalis, common in the 

 Mediterranean. 



The Cephalopoda, or at any rate the Dibranchia, have a more com- 

 plete and perfect circulatory system than other Mollusca, the blood 

 being nearly entirely contained in true vessels. The heart is situated 

 somewhat superficially near the middle of the posterior or physio- 

 logically ventral surface. It lies in a pericardial cavity, except in the 

 Octopoda, in which it is much reduced. The essential part of the 

 heart is the median ventricle, the lateral and symmetrical auricles 

 being nothing more than simple contractile expansions of the efferent 

 branchial vessels. 



The branchiae, or ctenidia, are situated in the pallial cavity on either 

 side of the visceral mass. They originate posteriorly in the embryo, 

 between the mantle and the foot, and afterwards sink in towards 

 the bottom of the pallial cavity, where their axes are inserted, 

 their free ends pointing towards the head. Nautilus, the only living 

 representative of the Tetrabranchia, has 2 pairs of branchiae. All 

 other Cephalopods have a single pair of branchiae, and thus con- 

 stitute the order Dibranchia. much richer in living species than the 

 Tetrabranchia. 



