234 LECTURES ON MOLLUSCA. 



hairy or knobbed ornaments, and produced into a tail; and side-flaps 

 to the foot which may be used for swimming. When disturbed, they 

 discharge a beautiful violet fluid from the skin. The harmless "Un- 

 wasliables " (Aplysia) were formerly dreaded by fishermen, who thought 

 their stains were poisonous and indelible. They have a convex, horny 

 plate covering the gills; and sometimes. old Sea-hares have several of 

 these, one inside the other, as in the Cuttle-pens. They have a cartil- 

 aginous gizzard, like the Bubbles. In Syiohonota there is an excretory 

 tube above the tail. Dolabella has the plate shelly, and generally ax- 

 shaped. 



Aclesia is like Aplysia, without shell or swimming flaps. In Sty- 

 locheilus the neck and tail are very slender. Notarchus has the body 

 rounded, with a very narrow foot for adhering to floating sea-weed. 

 Bursatella presents a most anomalous appearance. The common ob- 

 server might take it for a jelly fish: for it is quite round, with only a 

 rudimentary foot, and with a mass of branched ornament. This con- 

 sists, however, first of a large gill hanging out of the back; and sec- 

 ondly, of the tentacles which are cut up into branches. 



Family Icaridje. 



A small family of Sea-slugs have a Bulloid shell, not covered by the 

 mantle, and only two stumpy tentacles, instead of four, as in Aplysia. 

 The body is thin, with a very long tail. The shell of Icarus resembles 

 Amphisphyra, with a notch at the suture. Lobiger has a thin shell 

 shaped like Pedicularia, with four spreading foot-laps, adapted for 

 swimming, like the Pteropods. 



The remaining families differ from the Bubbles and Sea-hares, in 

 having the reproductive organs close together, in one tubercle. 



Family Pleurobranchid^e. 



These animals have four stomachs, but very short intestinal canal. 

 They are sluggish, compact, often large, and have a somewhat re- 

 tractile proboscis. The head is hidden under the edge of the mantle, 

 with two tentacles and eyes. The gill is at the side, not on the back 

 as in Aplysia. Pleurobranchus has a thin, flat horny shield, and a 

 very large foot. The mantle in Oscanius is irregularly expanded, and 

 the shield silvery. Sicsania has a plain body, with very small shield, 

 and a large mantle deeply notched in front. 



Pleurobranchoea and Neda have no shield, alid a very small mantle. 

 The former has a narrow, the latter a broad foot. 



Family UMBRELLiDiE. {Chinese Umbrella Shells.) 



Again we come unexpectedly on a group of Limpets ; for so the 

 shells might be considered. The Umbrellas are very large creatures, 

 wearing a flat limpet on the middle of the back; not immersed in the 

 mantle, as in the very differently organized Lucapina. The gill is 

 below the shell, on the right side. The foot is enormously large, and 

 encloses not only the body but the head, which has a retractile snout. 

 Fossil specimens have been found in the Eocene beds. The animal of 



