DiaESTTVE ORGANS. 29 



teuthis, Eiioplotouthis, Ccla'no and Belemnites have iiooks, in 

 two rows ; and in the two former there is additionally a group 

 of small cupules and tiibercles at the hase which may bo used to 

 form by the apposition of these parts in the two tentacles a sort, 

 of fleshy articulation and sup]iort for the action of the armed 

 clubs. Chiroteuthis has an additional oval fleshy cupule at the 

 extremity of the club. 



The web which connects the arms in many of tlie cephalopods 

 may be likened to an umbrella of which the arms themselves 

 may represent the ribs. It is but little developed in some genera, 

 but attains an enormous development in Cirroteuthis, where it 

 unites all the arms to nearly their tips ; and in species of Trem- 

 octopus it is only developed between the dorsal or superior arras. 



The modification of one of the sessile arms of the male Cephal- 

 opods for sexual purposes, causing it to assume a totally ditterent 

 appearance, will be more appropriately treated under .vernal 

 organfi. 



In the tetrabranchiates, of wliich the Nautilus is an example, 

 the foot is represented b}^ a sheath, in the margin of which are 

 digitations, eight in number, and from these digitations project 

 in a dou1)le series, thirty-six small unarmed brachial tentacles, 

 lamellated on their inner surface, and retractile at the will of the 

 animal. This sheath expands greatly dorsally, forming a tri- 

 angular, tuberculate, fleshy hood, by which the aperture of the 

 shell may be closed in lieu of an operculum. In addition to the 

 brachial there are other tentacles; namely four groups of twelve 

 or thirteen each, termed labial, and surrounding the lips, and 

 four ocular tentacles, situated one in front and one behind each 

 eye, and which appear to be organs of sensation. In the male 

 there is some modification ; the internal tentaculiferous lobes 

 are wanting, and the external ones are divided into an anterior 

 with eight and a posterior one with four tentacula. Upon the 

 left side, moreover, the four posterior tentacles are modified into 

 a peculiar hectocotyle termed a sjMcli.r, bearing a discoidal fol- 

 licular gland upon its outer surface. 



Digestive Organs. 



The organs of manducation are composed in the cephalopods 

 of a beak in the form of two corneous or calcareous mandibles, 



