296 



LOWER INVERTEBRATES. 



Fig. 334. — Doris bifida, showiiii 

 gills and a tentiicle enlarge) 



the 



it turns and swims as actively as any other gasteropod which retains its calcareous 

 armor throughout life. 



The adult, however, is not always without protection other than that afforded by 

 its resemblance to the objects which it frequents, for in some forms the mantle secretes 

 calcareous spicules of various shapes, which sometimes are so numerous and so inter- 

 twined that, when the fleshy parts are dissolved in caustic 

 potash, they retain the positions they occupied when in 

 life. When these spicules are numerous, they cause the 

 dorsal surface to be roughened and hardened, forming a 

 protective dorsal shield. 



As is implied in the foregoing, a mantle is sometimes 

 present, but in others this structure is not differentiated. 

 When present it is perforated, and through the openings 

 project the tentacles and the gills. In the young, well- 

 developed eyes are present, but in the adults they appear 

 as minute black dots, just behind the tentacles, or are ob- 

 solete. The tentacles are prominent, and seem to serve as 

 olfactory organs, and not as organs of touch. They are 

 frequently made up of a series of plates, presenting an 

 appearance which recalls the antennas of many insects ; 

 at other times they are plaited or simple, and not infre- 

 quently they may be retracted into trumpet-shaped sheaths near the base. These vari- 

 ations are of much importance in systematic work. 



The gills, as we have said, are typically not inclosed in a cavity of the mantle, but, 

 when present, they project freely into the sea. They vary greatly in form and dispo- 

 sition, furnishing, in these respects, important systematic characters. Sometimes they 

 are in a more or less complete circle, surrounding the posterior opening of the aliment- 

 aiy canal, or they may be arranged in longitudinal series along the sides of the back or 

 body; in the PhyllididEe alone do we find any approach to the formation of a branchial 

 sac. In form they may resemble bushes, or they may be reduced to simple papilla ; 

 all variations between these extremes being found. These gills perform but a part of 

 the respiratory economy, for, in all, the general surface of the body serves for the 

 aeration of the blood, and in the forms without gills it is the sole agent in this process. 

 The forms with gills are said to flourish when deprived by accident of these organs, 

 the skin performing their functions. 



In the internal anatomy we find some points which deserve a brief mention. The 

 lingual ribbon varies in the number and arrangement of the teeth, according to the 

 family. The alimentary canal usually terminates on the right side of the body, though 

 in forms like Doris it may end medially. The stomach is surrounded by a large, 

 much-branched liver, portions of which extend into the elongated papillas, which are 

 found on the back. In the apices of these papillsB are found thread cells, recalling the 

 similar defensive organs of the Hydrozoa. 



The nudibranchs are mostly littoral forms, and spend their lives creeping among 

 the rocks and seaweeds near the shores. They can, however, swim, and, when em- 

 ploying this mode of locomotion, they usually progress with the back downward and 

 the foot uppermost. The food may be either vegetable or animal. Some forms feed 

 on the more minute algae, while others create sad havoc among the hydroids. The 

 eggs are laid in bunches, upon stones, hydroids, or sea-weeds, almost every species 



