22 * NUDIBRANCHIATA. 



is always stiffened by imbedded calcareous spicula of 

 various forms ; and its outer surface, in all tlie Nudi- 

 branchs, is provided with vibratile cilia. Their habits 

 are very interesting. Some kinds are nocturnal ; others 

 may be observed in a state of activity during the day- 

 time, in tide-pools left among the rocks and in shallow 

 water, apparently enjoying the warm rays of the sun; 

 the greater number avoid the light, concealing them- 

 selves under stones and shelving rocks. They range 

 from a little below high-water mark to nearly one hun- 

 dred fathoms on our northern coasts. They are usually 

 sluggish in their movements, some remaining for hours 

 fixed to a spot ; a few, however, are very active and 

 lively. Crawling is the ordinary mode of progression. 

 '^ This is effected, in the manner of the snail, by a series 

 of minute undulations of the under siu'face of the foot, 

 arising from the alternate relaxation and contraction of 

 the pedal muscles.'^ They also float slowly along the 

 surface of the water, in an inverted position, probably by 

 an action similar to that of crawling, aided by the slime 

 which continually exudes from the body, and on which it 

 appears to creep. While thus floating, ^Hhe Nudibranchs 

 occasionally drop suddenly down, suspending themselves 

 from the surface by a thread of mucus, which is fixed 

 to the tail or posterior extremity of the foot. In 

 this way they will let themselves gradually down to the 

 bottom, or remain some time pendent in the water 

 without apparent support ; for the thread of mucus is 

 so transparent that it can scarcely be seen.^^ It is said 

 that some kinds (e. g. Tethys) swim freely through the 

 water in any direction; these have a large head-veil 

 and crested tail, which serve as fins. The Rev. 11. T. 

 Lowe, in his graphic description of the genus Peplidia, 

 found on the shores of Madeira, says (Ann. & Mag. 



