210 INVERTEBRATE ANIMALS. 



the beak of a Lird (fig. 149 m), together with an " odontophore " 

 or " tongue," the hinder part of "which is furnished with recurved 

 spines. This cavity leads by a gullet, furnished with salivary glands, 

 into a stomach, from which an intestine is continued to terminate by 

 a distinct anus, which opens on the ventral sui-face at the base of the 

 so-called " funnel." The funnel is a muscular tube placed on the 

 under surface of the head, and communicating on the one hand with 

 the external medium, and on the other with the cavity of the mantle. 

 In the jVavtihis alone it is simply formed of two muscular lobes, 

 which are in api>o.sition, but are not united together so as to form a 

 tube. In many cases there is also a special gland, known as the 

 "ink-bag" (fig. 149, i), for the secretion of an inky fluid, which the 

 animal discharges into the water, so as to enable it to escape when 

 menaced or pursued. The duct of the ink-ljag opens at the base of 

 the funnel near the anus ; but the Pearly Nautilus and the allied 

 fossil forms are without this means of defence, which the jjresence of 

 an external shell renders unnecessary. 



The respii'atory organs ai'e in the form of plume-like gills, placed 

 on the sides of the body in a branchial chamber, ■which opens in 

 front on the under surface of the body. In almost all the living 

 C'ephcdopodii, there are only two gills, one on each side, and hence 

 this section is known as that of the ^'' JJiJifunrliiatu.'" In the Pearly 

 Nautilus alone there are four gills, two on each side, hence the name 

 of " Taruhmiii'liiata" applied to the order of which this is the only 

 living representative. In the C'uttle-fishes, at the base of each gill 

 is a .special contractile cavity, called a "branchial heart," by which 

 the venous bl(jod, returned from the body, is driven through the 

 gills. In addition to these branchial hearts theie is a true arterial 

 heart, by which the aerated blood received from the gills is driven 

 through the Ijudy. The aibnission of water to the branchi;e is 

 effected by the eV^pansion of the mantle, which allows the entrance 

 of the outer water into the mantle-cavity. The mantle then con- 

 tracts, and the water is ff)rcibly expelled through the funnel, which 

 is often furnished with a valve, allowing the passage of water out- 

 wards, but preventing its entrance inwards. By a rejjetition of this 

 process both respiration and locomotion are simultaneously effected, 

 for the jets of water exiielled from the funnel by their reaction 

 drive the animal in the opposite direction. In this case, therefore, 

 as in many others, the more active the animal is, the more perfectly 

 is the respiratnry process carried on. 



The nervous system is foi'med U]3on essentially the same plan 

 as in the other Mollum'a, biit the great nerve-centres are col- 

 lected to form a ring round the gullet, and are protected by a 

 cartilage, whicli is to be reganled as a j-udimentary skull. This 



