ORGANS OP MOVEMENT IN THE GASTROPODA, ETC. 05 



of' Treraoctopus it is only developed between the dorsal or 

 superior arms (xxiii, 8). 



The Biodification of one of the sessile arms of the male 

 Cephalopods for sexual purposes, causing it to assume a totally 

 different appearance, will be more appropriately treated under 

 sexual organs. 



The tetrabranchiates, of which the Nautilus is an example, 

 (iv, 62), develop a sheath^ in the margin of which are digitations, 

 eight in number, and from these digitations project in a double 

 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 triangular, tubereu- 

 late, 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 tenta- 

 cles, situated one in front and one behind each ej^e, 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 hecto- 

 cotyle termed a spadix, bearing a discoidal follicular gland upon 

 its outer surface. 



ORGANS OF MOVEMENT IN THE GASTROPODA, ETC. 



Foot. The ybo^ is a fleshy, expanded mass, forming the entire 

 under side of the body, or attached to it in front of the mantle 

 by a peduncle. In the heteropods, etc., the foot is divisible into 

 three portions, termed, respectively, propofZram, mesopodium and 

 metapodium ; but in most typical gastropods these three areas 

 are blended in the sole, although the metapodium is indicated 

 by the fact of its supporting on its dorsal side the operculum. 

 In Strombus (iii, 49), a transverse furrow separates the mesopo- 

 dium from the propodium, and the metapodium is covered 

 downw^ards and in front by the operculum. 



The peduncle of the foot, when differentiated, is usually short 

 and depressed, and covers the under side of the body between 

 the mantle collar and mouth, the foot being expanded forward, 

 but more extensively backwards ; but in Strombus and its allies 

 the foot is slim and cjdindrical. Haliotis, Patella and Chiton 

 have the foot, on the other hand, very much expanded. Rapidity 

 of .motion appears to be in inverse ratio to the size of the foot ; 

 those genera in which the organ is enormously developed, espe- 

 cially in those just cited, where it occupies the entire ventral 

 surface of the body, being slow in movement. 



Frequently the anterior border of the foot is variously lobed 



