324 



COMPARATIVE ANATOMY. 



Fig. 

 the 



171. Diagram of the relations of 

 mantle and foot ; vertical section. 

 A In Lamellibranchiata. J? In Cephalo- 

 pora. m Mantle, p Foot, br Branchiae. 



the reason of its being called a foot (Fig. 171, B). Sometimes, 

 however, it is elongated, and, at others, discoid in form. In most 

 of the Gastropoda the foot is only sharply marked off along its 

 plantar margin. In many of the lower Prosobranchiata (Haliotis) the 



surface of the body above 

 the foot is drawn out into an 

 encircling edge (epipodium), 

 which is distinguished from 

 the mantle by surrounding 

 the head. The foot of the 

 Heteropoda is 'differentiated 

 into a more independent organ, 

 which springs from the ventral 

 surface of the animal, and 

 forms a vertical fin. The body 

 is continued in front of, as 

 well as behind, the foot. This 

 arrangement is very different 

 from the primitive one; the body has no longer a flat surface, 

 although the end of the foot in Atlanta still carries an operculum. 

 The structure of the muscular sole of the Gastropod foot is retained 

 in rudiment as a sucker-like organ, which in the Pterotracheae is 

 found in the males only. And we are reminded by this that even 



when fully developed the foot of the 

 Gastropod functions as a sucker, for the 

 animal is able to attach itself by it. 



The modifications undergone by the 

 foot in the Pteropoda are still more 

 significant. The foot, which, in the ear- 

 liest larval stages, is formed in just the 

 same way as in the Gastropoda, gives 

 rise, in the Cymbulidas and Hyaleida3, to 

 a median and two lateral pieces (cf. Fig. 

 170, C pp). In the Hyaleidaa the median 

 portion is feebly developed, while the 

 lateral lobes become large fins, which 

 embrace the rudimentary head, just like 

 wings ; in the Cymbulida) the median 

 piece is also well developed. It either 

 fuses at its base only (Cymbulia), or along 

 its whole length (Tiedemannia), with the two lateral lobes ; in this 

 way the large fins of these animals are produced. 



Fig. 172. Diagram of the rcla. 

 tions of the mantle. A In 

 Pteropoda. B In Cephalo- 

 poda. p Foot. In- Branchire. 

 t Tentacles. 



§ 252. 



The greater development of the head in the Cejohalopoda is an 

 important peculiarity as affecting the form of the body, while the 

 mantle acquires the same relations as it has in the Thecosomatous 

 Pteropoda, from which therefore they may be derived. The cavity, 



