201 THE IMECIIANICAL rUNCTIONS. 



the marine muscles possess the art of tbrmiug these 

 tliieads from the earhest periods of their existence ; 

 for he saw them practising it, when the shells in 

 which they were inclosed were not larger than a 

 millet seed.* In Sicily, and other parts of the 

 Mediterranean, these threads have been manufac- 

 tured into gloves, and other articles, which resemble 

 silk. 



§ 3, Gasleropoda. 



The Mollusca which inhabit univalve or turbinated 

 shells belong to the order of Gasteropoda, and have 

 a more highly developed organization than the 

 Acephala. The part which performs the office of 

 a foot is a broad expansion of fleshy substance, 

 occupying nearly the whole under surface of the 

 animal, and forming a flat disk, capable of being 

 applied to the plane along which it moves. This 



is seen in the Plaiiorhis 

 (Fig. 105, d). In some 

 species it is fashioned into 

 a projecting ridge, which 

 cuts its way, like a plough- 

 share, along the surface on 

 which it moves. The bands of muscular fibres, 

 which compose the principal part of its structure, 

 are short, and are interlaced together in a very 

 intricate arrangement. All the columns of their 

 fibres terminate at the surface of the disk; so that 

 when the animal is crawling, their successive actions 



* Memoires de I'Academie des Sciences: 1711, p. 118 to 123. 

 Poll conceived that these threads are dried muscular fibres ; an opi- 

 nion which has been adopted by De Blainville. 



