116 



COMPARATIVE ANATOMY 



CHAP. 



the brachial ganglion, which lies under the oesophagus, and is an 

 anterior division of the pedal ganglion. (2) The arms do not 

 occupy, in the embryo, their definitive position round the mouth, but 



rise on the ventral side behind the 

 mouth, between it and the anus, in a 

 row on each side. These two rows shift 

 secondarily forward to form the circle of 

 arms round the mouth. (According to 

 another view, the arms are cephalic ap- 

 pendages, comparable with the cephalic 

 tentacles of the Pteropoda.) 



The pedal nature of the siphon or 

 funnel has rarely been doubted. It is 

 innervated from the pedal ganglion. Its 

 two lateral lobes, which in the Nautilus 

 remain separate throughout life, but in 

 the Dibranckiata overlap, may be con- 

 sidered as epipodia. The accompany- 

 ing figure of a Cephalopod embryo con- 

 FIO io3.-Embryo of a cephaiopod, fi i}{ ini the rad imentary 



seen obliquely from the left posterior side _ _ i . } . . , 



(after Grenadier). 1, Mantle ; -2, anus ; siphon is 86611 in the typical position of 



3, right ctenidium ; 4, rudimentary epipodia in the shape of two lateral folds 



siphon ; 5, auditory organ ; 0, arms ; 7, i_ i j i_ 1 j 



yolk-sac s left eye. running backward above the toot and 



under the visceral dome. 



In Nautilus and the Decapoda. (excluding the Loligopsidce) a valve 

 is present within the siphon. For the form of the siphon, see p. 38. 



1. The Arms of the Tetrabranchia (Nautilus). 



The "head" of the Nautilus (Fig. 104) carries numerous tentacles placed in*a 

 circle round the mouth ; these do not rise directly from the integument around the 

 mouth, but stand upon special lobes which are differently developed in the two 

 sexes. These lobes may be compared with the arms of the DibrancMa, and the 

 tentacles they carry, perhaps, with the suckers on those arms. Each tentacle can be 

 retracted into its own basal portion as into a sheath. 



If the head be viewed from the ventral side, so that the mouth appears lying in 

 the centre of the extended lobes and tentacles, we see in the female (lower figure) 

 three inner lobes close to the mouth, two lateral and one posterior. The posterior 

 inner lobe consists of two fused lateral lobes, the line of fusion being indicated by a 

 lamellated (olfactory ?) organ. It carries twenty-eight tentacles, fourteen on each side. 

 Each lateral inner lobe carries twelve tentacles. Besides these three inner lobes, 

 the foot develops a muscular circular fold ; this is particularly thick anteriorly, and 

 here forms a lobe, the so-called hood (Fig. 32, a, p. 22), which, when the head is 

 retracted, covers the aperture of the shell like an operculum. The outer circular 

 fold carries nineteen tentacles on each side. 



Besides these tentacles which belong to the foot, there are two more on each side 

 which probably belong to the head, one lying above and the other below the eye. 



In the male Nautilus (upper figure) the posterior inner lobe is rudimentary. 

 Each of the lateral inner lobes is divided into two portions. In the right lobe, the 

 anterior portion carries eight tentacles and the posterior (antispadix) four, three of 



