818 CLASS XIII. 



the cerebral ganglion, the lowest contains the auditory organs, and 

 the lateral parts, broad and excavated like a cup, receive the eyes. 

 The orbits are guarded in front by two elongated cartilaginous plates, 

 which are attached by their base to the cartilaginous ring and else- 

 where are extended freely over those cavities. In the Sepia there is 

 an additional triangular cartilage, of which the point is attached to the 

 ring, the base turned towards the arms, and to which the muscles 

 of the arms are affixed. In Sepia and Loligo there are further 

 found two catilaginous plates on the back, and, on each side of the 

 body, an elongated flat cartilage which supports the lateral expan- 

 sions of the mantle or the fins \ In the Nautilus there is only one 

 cartilage present in the head, which lies beneath the oesophagus, but 

 does not enclose this annularly, and is prolonged on each side to the 

 funnel. The eyes, which in this genus are placed on pedicles at the 

 side of the head, are not protected by this cartilage. 



The head of these molluscs is round and broad and separated by 

 a constriction, as by a neck, from the body. Around the mouth are 

 placed in most eight or ten arms; in Nautilus, on the contrary, 

 several membranous sheath-like appendages, perforated internally, 

 in which thin cylindrical tentacles, capable of extension and retrac- 

 tion, are inclosed. Where eight or ten arms are present, eight of 

 them have constantly a somewhat conical form and stand in a circle 

 round the mouth. On the upper surface, that which faces towards 

 the mouth, these eight arms are covered with numerous suckers, by 

 which they attach themselves to different bodies. (In the ten-armed 

 two are placed on the outside beyond the circle of the other eight on 

 the ventral surface, and can by retortion be drawn entirely within 

 the body. Already had ARISTOTLE distinguished these two from the 

 other arms under the name of proboscides.} The arms are hollow 

 internally, for the passage of the artery and nerve ; radiating mus- 

 cular fibres run from the middle outwards, and on the surface is a 

 layer of circular and longitudinal muscular fibres, by which all the 

 various motions of retraction, contraction, flexure and convolution 

 become possible, which, on the seizure of prey, are executed by 

 these organs. 



Under the head is situated a conical organ, which is open at the 

 extremity, the funnel (infundibulwm) formed by an extension of the 

 mantle and provided with muscular walls. In Nautilus it has two 



1 See C. A. SCHULTZE in MBCKEL'S Arrkiv fur die Physiologie, iv. 1818, s. 334 

 338, Tab. iv. fig. i. 



