The Opisthobranchia include the “Pteropods” formerly considered as a distinct class, the “Bubble-shells” (Bullidæ), the “Sea-Hares” (Aplysiidæ), the “Umbrella-shells” (Umbraculidæ), the Nudibranchs and some others.
Fig. 16.
Shell-bearing Pteropod (Cavolina tridentata). Case 96.
a. Shell and animal. b. Side view of shell. c. Dorsal view of shell.
The Pteropods[[4]] are sometimes called Sea-butterflies, and are organized for swimming freely in the ocean. They have a pair of fins developed from the sides of the mouth or neck, which perform a flapping movement during progression. Some Pteropods (Thecosomata) are provided with small glassy shells; others (Gymnosomota) are naked. They exist in countless millions in some parts of the ocean, discolouring the water for miles. They constitute the principal food of the Baleen Whales.
Fig. 17.
Shell-less Pteropod (Clione limacina).
a. Dorsal view. b. Ventral aspect.
About a hundred species are known.
Case 96.
The Sea-Hares, so called on account of a slight resemblance to a crouching hare and not for their nimbleness of foot, are found in most parts of the world, in pools at low water. At the hinder part of the back two flaps of the mantle partly conceal a thin horny shell which serves as a protection to the gills and vital organs beneath. When molested, these animals discharge a large quantity of a purple fluid, discolouring the surrounding water for a distance of more than a yard.
