jLpsil — sept. 1858.] Order lladiata. Ii29 



are more or less developed according to every variety of type, and 

 are very important in the classification of these animals. 



Many Molluscs remain fixed to the ground except during their 

 larval condition ; others creep by an extraordinary brgari woven 

 out of muscular threads which is called a foot ; many swim, some 

 by sucking in water and then expelling it by a rapid contraction 

 of the body ; others by actual swimming organs which sometimes 

 consist of rows of membranes covered with bristle like hairs, and 

 sometimes of swimming-flaps, attached to various parts of the body. 



The circulation of the blood is in a high state of development in 

 most Molluscs, for they often possess a heart divided into several 

 chambers, with both veins and arteries spreading though the whole 

 body ; but this as well as the breathing organs vary much in de- 

 velopment in the different classes. 



The greater number of Molluscs live in the water, the lower 

 classes almost all in the sea, and it is only the higher classes which 

 possess some genera living exclusively on land : — even these earth 

 inhabitants however prefer damp swampy places and are destroy- 

 ed by complete drought. The creeping genera prefer the lower 

 surface of stones and water-plants, many bore into sand and still, 

 even wood and limestone are not safe from their attacks. The ac- 

 tual swimmers all live in the sea, and are often met with on the 

 open ocean. 



The Molluscs belong to the first inhabitants of our earthly ball, 

 and by the strength of their shells which have been preserved in 

 large masses, have done great service to Geology. We shall only 

 be able properly to consider their development in the different 

 strata, when we have learned thoroughly to understand the pecu- 

 liarities of the different classes and families. 



Sub kingdom o£ Molluscoids. 



The Molluscoids distinguish themselves by their very unsymme- 

 tncal form, and by only possessing one ganglion, which is gene- 

 rally connected with an eye-like organ placed immediately upon 

 it. A separate head is never found amongst them, for even in those 

 classes which like the « moss-animals" (Bryozoa) possess a mouth 

 opening surrounded by feelers, there is never any division between 

 Vol. xx o. s. Vol. iv. n. s. 



