44 



VI. — Miscellanea Zoologica. By George Johnston, M.D., 

 Fellow of the Royal College of Surgeons of Edinburgh. 

 Plates II. and III. 



(Continued from Magazine of Zoology and Botany, ii. p. 73.) 

 IV. — The Scottish Mollusca Nudibranchia. 

 Character. Mollusca gastropodous, shelless, bisexous : 

 branchice external, dorsal, always symmetrical, placed either 

 posteriorly in the median line or along the sides : tentacula 

 from one to three pairs, more or less retractile : eyes sessile or 

 none : head scarcely marked. Nudibranches, Cuvier,Reg. 

 Anim. iii. 50. Rang, Manuel, 124. — Tritoniens, Lamarck, 

 Anim. s. Vert, vi. i. 298. 



The Nudibranchia are all natives of the sea, where they re- 

 present the land slugs, to which they have a considerable degree 

 of outward resemblance, and are otherwise allied to them in 

 structure and habits. Like the slugs they crawl on a muscular 

 disk or foot which occupies the entire length of the body ; their 

 mode of progression is of the same slow and continuous na- 

 ture ; a glairy fluid exudes from miliary glands imbedded in 

 the skin to render the surface lubricous, less resistent, and 

 less liable to injury from friction ; both tribes are equally vege- 

 table feeders ; and they are also hermaphrodites after the same 

 remarkable fashion. What peculiarly distinguishes the pre- 

 sent order is the external position of the branchiae or breath- 

 ing organs, where placed on the back they float in the oxygen- 

 ating medium, and require no subsidiary apparatus to bring 

 this into contact with the blood. In form and location these 

 organs are exceedingly diversified, becoming, when fully dis- 

 played, the creature* s principal ornament, and its most obvious 

 claim to our notice and admiration. They are placed on the 

 posterior part of the body in the Dorides, where they form a 

 circle of arborescent leaflets, which can be drawn within at 

 pleasure and removed from injury ; but in all the other genera 

 they are incapable of retraction, and are distributed on the 

 sides and over the back in a pattern each after its kind. To 

 aid them in their important function, we find that the surface 

 of these branchial leaflets or filaments is clothed with minute 

 cilia, which by a quick and constant succession of vibrations in 



