390 NUDIBRANCHIATA. HETEROPODA. 



longations of the stomach and liver. In the Glaucus (Fig. 681), 

 they form two or three large tufts on each side, which give as- 

 sistance in swimming. These beautiful little MoUusks are in- 

 habitants of the Mediterranean and Indian Oceans ; their hues 

 are azure blue and silver. The small family of the ELYSIAD^E 

 consists of a few minute slug-like animals, in which no trace of 

 special respiratory organs have been detected. They appear to 

 feed upon Sea-weeds. % 



1007. Most of the Nudibranchiata deposit their eggs on the 

 shore in gelatinous masses ; the eggs being very regularly ar- 

 ranged 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 3-1000ths 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. ascer- 

 tained that there must have been, in the whole mass, at least six 

 hundred thousand eggs. 



ORDER IV. HETEROPODA. 



1008. The Mollusks of this Order, which have been regarded 

 by some Zoologists as forming a class distinct from the Gastero- 

 poda, are distinguished from all others by their complete adaptation 

 to an oceanic mode of existence. Instead of creeping about upon 

 rocks and sea-weeds at' moderate depths near the shore, they 

 swim about freely in the open sea, often displaying great activity 

 in their movements. To fit them for this peculiar mode of life, 

 the whole body is much compressed ; the foot usually forms a 

 vertical muscular paddle, serving as a fin, whilst the hindermost 



