212 



INVERTEBRATE MORPHOLOGY. 



stoiies during the day-time, but becoming, in some species at 

 least, free-swimming at night ; Lepidonotus, characterized by 

 the possession of elytra arranged in overlapping series on the 

 dorsal surface ; Diopatra, which forms tubes for itself by glu- 

 ing together particles of foreign matter ; and Autolytus and 

 Syllis, which are peculiarly pelagic in habit, as is also Alciope, 

 characterized by its large highly-organized eyes. 



3. Order Sedentaria. 



This order includes a number of forms which manufacture 

 for themselves tubes of various substances some being 

 merely composed of particles of sand glued together by an 

 adhesive secretion, while others consist of a chitiuous sub- 

 stance, to which foreign bodies may be added, or even of car- 



FIG. 100. ANTERIOR 

 END OP Nereis virens. 

 c cirrus. 

 p = parapodium. 

 pi = palp. 

 t = tentacle. 



FIG. 101. Amphitrite ornata (after 



VERRILL). 



bonate of lime. Within these tubes the animals permanently 

 reside, and in conformity with this mode of life numerous 

 adaptations of structure are found. The head is usually pro- 

 vided with a number of long cirri and the branchiae are for the 

 most part confined to the head region. In some forms, such 



