MOLLUSCA 



263 



Class III. Cephalopoda (^head-footed; Squid, Devil-fish). — The cephalopods 

 are bilaterally symmetrical moUusks with a well-developed head in which the front 

 part of the foot surrounds the well-armed mouth as a series of lobes or tentacles. 

 The head protrudes permanently from the mantle cavity, leaving the mantle 

 surrounding the posterior part of the body. The posterior lobe of the foot forms 

 a siphon, communicating with the mantle cavity. Into this cavity the nephridia, 

 the anus, and the reproductive glands open, and in it the gills lie. The shell may 

 be present and external {Nautilus), internal and slightly developed (Squid), or 

 wanting (Octopus). An internal cartilaginous skeleton protects the brain. The 

 coelom is well developed. The ganglia of the nervous system are massed in the 

 head region. The sexes are separate and the development direct. The Cephalo- 

 poda are to be looked upon as the most highly developed of the Mollusca. They 

 are little in evidence now, however, as compared with earlier times. 



Pig. 124. 



Fig. 124. The Devil-fish (Octopus). From Cooke, after Merculiano. A, at rest; B, swimming, 

 a, arms, with suckers on the inner aspect; e. eye; s. siphon or funnel. 



Questions on the figure. — Which is the anterior end of the animal? What 

 is the position of the mouth? What is the function of the siphon? Of what 

 structure is it a part? 



Subclass I. Tetrabranchiata. — Cephalopoda in which the front segment of the 

 foot is divided into lobes bearing numerous tentacles, without suckers. Shells 

 external and chambered (and in Nautilus, the only living genus, coiled). Two 

 pairs of auricles; two pairs of gills; two pairs of nephridia. 



This group is important for its extinct rather than for its living representatives. 

 The pearly or chambered nautilus (Pig. 123) found in the Pacific and Indian 

 Oceans, is the only important living species. The Nautilus appears to be the only 

 remaining representative of the orice numerous coiled forms and more remotely 

 still of the Orthoceratites, the rulers of the Palaeozoic seas (see Geology). 



Subclass II. Dibranchiata. — Cephalopods in which a circlet of 8 to 10 arms 

 surround the mouth. These bear sucking discs. Shell internal and rudimentary 



