HARDWICKE'S SCIENCE-GOSSIP. 



i8i 



existence. Most people know what shells are, and 

 not a few epicures smack their lips right heartily on 

 devouring the contents of some of them ; but very 

 few have the least suspicion of the reality of a tribe of 

 creatures which, although comparatively insignificant 

 in the magnificent repertory of marine animals, yield 

 to none in respect to beauty of appearance and in 

 delicacy, subtlety, and harmony of anatomical 

 structure. 



Frequently, when traversing the rough, rocky, weed- 

 carpeted space that intervenes between high and low- 

 water mark, we may observe a curious pale-orange 

 coloured body like the frizzled half of a slightly 

 squeezed lemon, and on taking it up and placing it 



behind, supply the skin of the back and the gills ; 

 (3) the pedal, situated at the sides, which supply the 

 whole foot. Besides these three, [there are the olfactory, 

 the optic, the buccal, the visceral ganglia, which are 

 small, making in all about seven pairs of excito-motor 

 ganglia and a single one, which'send off about twenty 

 pairs of nerves and four single ones to the various 

 organs. In addition to these, therejis a sympathetic 

 system of nerves, which is extensively and beautifully 

 distributed over the viscera, and is united by nerves 

 forming plexuses and connected in front with the 

 buccal and branchial centres, but there is no special 

 relation between it and the blood-vessels, such as 

 exists in the higher vertebrata. As regards the senses. 



Fig. 127.— Pal.ite of Eolis Dnanmandi ( X 100). a, side view ; 

 /', front view. 





Fig. 128.— Thread-cells from Papilla of Eolis Drummondi 

 (X 400-800), drawn by the Author, a, the thread contracted ; 

 b, the thread discharged. 



Fig. 130. — Doris bilamel- 

 lata. Natural size. 



Fig. 129. — General structure of eye of Nudibranch. a, optic 

 ganglion ; b, optic nerve ; f, general capsule ; g, choroid ; 

 d, lens ; e, cornea. Optic nerve traverses pigment layer t» 

 reach "rods and cones " in front of retina. 



Fig. 131. — Shells of embryo Doris. X 100. 



B.^- 



Fig. 132. — Doris tuberadata. Natural size. 



in a vessel of sea- water, behold a magic change ! 

 The half-lemon swells into "ship shape," and from 

 near one extremity two laminated horns emerge, 

 while from the other end an exquisite fresh-looking 

 tuft of feathers gradually unfolds to view. This 

 sedentary and sluggish organism is the Do7-is 

 tiibercitlata of science. An anatomical dissection 

 reveals the various structures which compose its 

 vital machinery. In the first place, the excito-motor 

 nervous centres are concentrated above the throat ; 

 three pairs of these are larger than the rest, viz. (i) 

 the cerebroid ganglia, which are the most anterior in 

 situation, and supply nerves to the tentacles, the 

 mouth, and the lips ; {2) the branchial, situated 



it may be observed, that, in general, the nudibranchiate 

 Molkisca are very sensitive to external influences, i.e., 

 they are well endowedjwith general sensibility; the 

 eyes, which, as seen in the young, are minute, immersed 

 black dots behind the tentacles, are probably only 

 sensible of light and darkness, not of colour or form ;. 

 the dorsal tentacles are probably organs of some sense 

 like our sense of smell ; the oral tentacles and the lips 

 are specially sensitive in all that regards the selection of 

 food ; and there is an apparent sense of hearing, the 

 nerves of which seem to arise from the cerebroid 

 ganglia. 



The heart is situated near the upper surface of the 

 body. It is contained in an envelope whereto is 



