APRIL — SEFT. 1858.] Order Radiata. 



139 



seen, and many kinds come to the surface only on the darkest 

 nights. Their swimming flaps are in constant swinging motion 

 like the wings of a butterfly, but at the slightest danger they draw 

 them in, and sink back into the dark abyss. The greater number 

 of them possess a very thin horny shell which encloses the whole 

 body of the animal, except the mouth and the swimming flaps. 

 Some of them are quite naked and then they have a firmer skin. 



These animals show the low grade of their organization in all 

 their organs without exception. The greater number have no re- 

 gular head and therefore neither feelers nor eyes : — only two genera 

 possess eyes, in the others they are altogether wanting, though hear- 

 ing organs are pretty generally developed. The mouth is general- 

 ly unarmed, often surrounded by short prolongations which some- 

 times bear sucking cups. Their anatomy and development are so 

 little known that we cannot arrange this sub-class property into 

 orders. 



We distinguish two families. The " Crystal-snails" {Hyalida) 

 possess a shell, sometimes spindle, sometimes shoe-shaped, some- 

 times spiral, and of extreme tenuity and bitterness, which causes 

 it to fetch a very high price with collectors : — feelers and eyes are 

 altogether wanting and no distinct head is to be perceived : see 

 fig. 11, plate vi. The mouth is to be found between the two 

 swimming-flaps, which stand horizontally on each side, and can 

 generally be drawn back into the shell. They live in all seas, 

 rising to the surface in swarms in the evening. Shells belonging 

 to this family have been found in the Tertiary strata, but whether 

 the so-called concelarias which are to be met with in great num- 

 bers in the transition formation really belong to this family is very 

 doubtful. Hyalea, Limacina, Cymbulia, Cleodora, Euryhia. 



The family of the " whale-snails" ( Clioida) is divided from the 

 former by the want of a shell, the possession of a distinct head ge- 

 nerally of feelers and in one genera of eyes. Those which abound 

 in the North South Polar seas, form a principal part of the food of 

 the whale, and as the whole little animal is scarcely an inch long, 

 it may readily be imagined what enormous masses of them are re 

 quired to feed their gigantic enemy, Clio, Pneumodenna. 



