MOLLUSCOUS ANIMALS, 2/ 



was influenced by the resemblance of tbe nudibrancliiates to the naked 

 air breathers known as slugs; but though these sea-slugs form a very 

 distinct and natural group, which I cannot help thinking require to be 

 kept separate as an order, the great difference both in their aerative ap- 

 paratus and their habits of life should prevent these two orders being 

 brought near to one another, and it seems probable that they will 

 •ultimately take their places in very different parts of the system. The 

 3rd order Inferobranchiata, distinguished from the preceding by the 

 branchiae being arranged in two rows under the projecting border of 

 the mantle instead of on the back, bears a close resemblance to it in 

 the form of the animals, but conforms essentially to the character of 

 the next order, of which more recent Malacologists make it the last 

 family. 4. The order called Tectibranchiata is known by the branchiae, 

 more or less divided, but not symmetrical, situated along the right side 

 or on the back, being covered by the mantle, which usually encloses 

 a shell. The animals are marine and like the preceeding orders are 

 hermaphrodite. The 5th order has been accounted a class under the 

 name Heteropoda, but certainly presents only a deviative form of 

 Gasteropoda, and as an order of that claps has been named Nucleo- 

 branchiata. It includes swimming mollusks with the foot converted 

 into a sort of fin, and the branchiaa consisting of feathery lobes on 

 the posterior and left side of the back, with the heart, liver, and other 

 viscera behind them in a common enclosuie. 



The 6th order is named Pectinibranchiata and is by far the most 

 numerous of the whole, embracing nearly all those which have spiral 

 shells of one piece and many with simply conical shells. The branchia3 

 composed of numerous segments ranged like the teeth of a comb, are 

 attached in one or several lines to the lower surface of a cavity forming 

 an opening between tho border of the mantle and the body, and 

 occupying the last whorl of the shell. 



Order 7th Tubulibranchiata chiefly differs from the preceding in the 

 animals being fixed to their place and hermaphrodite, whilst the sexes 

 are distinct in Pectinibranchiata, but recent authorities with obvious 

 propriety refuse to account these separate orders. 



Order 8. Scutibranchiata is really distinguished chiefly by the sexes 

 being united in the same individual, whilst the shell is widely expanded 

 without an operculum. The members of the order as arranged by 

 Cuvier are not very closely related, and this order also may be properly 

 combined with Pectinibranchiata. 



