60 TESTACEA. 



position by a strong muscular spring, wherein the foot is the most effec- 

 tive instrument. 



Only a few of these animals exhibit the restlessness of many occupy- 

 ing the univalve shells, which advance with considerable expedition, and 

 seem unwilling to remain long in any position. 



But several enjoy the peculiar property of forming real threads, by 

 which, as by so many cables, they anchor themselves permanently and 

 securely to the same spot at will. 



Thus, the locomotive faculty is, for the most part, very feebly and 

 unequally dispensed. In some, besides, the portion protruding is so soft, 

 so void of consistence, or so small, that, although inactive, it can be of 

 very little use. 



It has been already remarked that a glutinous matter lubricates the 

 body of many of the softer animals. The purpose of this, in its simplest 

 application, seems only for a protection from abrasion of the skin, by 

 the indurated substances and uneven surfaces among which they dwell. 

 Some aquatic animals appear as if suspended by an invisible thread amidst 

 their element ; the slightest sheath invests the body of others, serving as 

 a place of retreat, or for the shelter of others, and which is often secured 

 by adhesion to solid substances. Some of the bivalves in particular are 

 capable of producing quantities, smaller or greater, of strong conspicuous 

 white, brown, or black thread, which may be compared to the work of 

 some insects, especially the silk worm, as preparatory to quitting their 

 imperfect stages. 



By means of the secretory properties possessed by some of the lower 

 animals, silky threads are extended, technically called the byssus, serving 

 to .secure them in the position they have selected. 



This substance is various in quantity, colour, and tenacity. Some 

 of the Pectens form a small, short, whitish bysstis of weaker texture. 

 That of the Mijtili is brownish or blackish, larger and stronger. A bunch 

 of stout coarse black hair seems to secure the Modiolus to its site, thence 

 considerable force is necessary for detaching it. The threads of the Injssus, 

 is.suing from the middle of the body, within the shell, are conducted to a 

 certain spot by the organ which we compare to a foot, where it is affixed 



