DORIS. INFEROBRANCHIATA. TECTIBRANCHIATA. 375 



In the Eolis (Fig. 557), the act of respiration appears to be per- 

 formed by means of numerous finger-like processes, with which 

 the back is covered. Most of these animals deposit their eggs 

 on the shore in gelatinous masses ; the eggs being very regularly 

 arranged in rows or bands. Of the immense number of eggs 

 which some of these contain, an idea may be derived from the 

 following fact, which is mentioned by Mr. Darwin, in regard to 

 the mass deposited by a large white species about 3J inches long, 

 on the shores of the Falkland Islands. From two to five eggs 

 (each of them 3-lOOOths of an inch in diameter) were contained 

 in a little spherical case. These cases were arranged two deep 

 in rows ; and these rows adhered to each other in such a manner 

 as to form a ribbon, running across it from one edge to the 

 other. One of these ribbons measured twenty inches in length, 

 and half an inch in breadth ; and by counting the number of 

 the spherical cases contained in one-tenth of an inch of each 

 row, and the number of rows in an equal length of the ribbon, 

 Mr. D. ascertained that there must have been, in the whole 

 mass, at least six hundred thousand eggs. 



ORDER III. INFEROBRANCHIATA. 



916. The small number of Mollusks contained in this Order 

 differ but little from the last, except in the position of their gills, 

 and their incapability of swimming. They are, therefore, con- 

 fined to the sea-shore; where they subsist (as do also the 

 Nudibranchiata) upon sea-weeds and other aquatic plants. 



ORDER IV. TECTIBRANCHIATA. 



917. This Order begins to show an approach to that 

 arrangement of the gills, which characterises the great bulk of 

 the Class ; these organs being concealed beneath a fold of 

 the mantle, in which a small shell is usually contained. The 

 animals composing it are all marine ; and live chiefly on the 



