26 NUDIBRANCHIATA. 



Gosse, and Stretliill AYright supposed that they are 

 adveutitious, being faecal and derived from tlie hydroid 

 polyps and Actiniae on Ayhicli the Eolides subsist. Alder 

 and Bergh, on the contrary, believed that the filaments 

 are really the product of the Eolides, and used by 

 them as weapons of defence. In some genera of 

 this family they are altogether wanting. According to 

 Dr. Baur^ that abnormal and puzzling mollusk Ento- 

 concha mirabilis of the late J. Miiller (which is an 

 internal parasite of Synapta digit at a) is a Nudibranch. 

 Although most of this Order are zoophagous^ Lima- 

 pontia and others of a simpler kind feed on seaweeds, 

 and are preyed on by Crustacea and small fishes. It 

 seems as if one of the principal uses of all animals were 

 to eat or to be eaten. Let us apply this idea to our own 

 case. What a gloomy prospect would lie before us^ 

 if there were no hereafter, and we could only say of our 

 physical destiny, 



Every organic thing we see, that springs 



To sentient life, by the Creator's will, 



Its individual ofEce to fulfil, 

 To us a sad and warning message brings. 

 'Twixt fear and hope, in war or doubtful peace, 



Each for its own existence ever strives : 



Until at last the fatal hour arrives. 

 When fear and hope, and life itself, shall cease. 



The leading genera have been well characterized by 

 Dr. Johnston in the following terms : — " The Doris has 

 its branchise, sometimes feathered like an ostrich-plume^ 

 placed near the posterior extremity; and the creature 

 has the power of concealing them when in danger. 

 The Tritonia is more slug-like than the Doris; but 

 its branchise form a curled fringe, interrupted at inter- 

 vals, along each side of the back. The Tethys has, 

 besides, a largely expanded veil over the mantle, im- 



