3324 THE MOLLUSKS OR SHELLFISH 



in the mantle some distance from the anterior extremity of the body. They are 

 smooth at the base, but lamellated toward the tip. The eyes are minute and sit- 

 uated in the skin behind the tentacles. 



A large number of species from all parts of the world, at one time 

 considered forms of the old genus Doris, are now placed in the distinct 

 group Doridopsidce , on account of differences in the oral organs. The general 

 aspect of the animals of this family is the same as that of Doris, the gills being 

 arranged in a circle round the vent on the posterior part of the back. The buccal 

 mass or eating apparatus is small, without jaws or radula, and organized for suc- 

 tion. The extensive family Dorididce contains a considerable number of genera 

 and subgenera, characterized by differences in the branchiae, the relative proportion 

 of the mantle in regard to the foot, and variations in the radula and jaws. The 

 general aspect of the genus Doris is represented by the figure here given of 

 Acanthodoris pilosa. The species are very numerous, and some of them are among 

 the largest of all Nudibranchs. They are world wide in their distribution, over 

 twenty species occurring in British seas. When living, they are most beautiful 

 objects, and must be studied in that state, for when dead and immersed in spirit, 

 they so contract that their natural form can only be guessed, and all the beautiful 

 and varied colors disappear. The spawn of Doris and other Nudibranchs is 

 deposited in the shape of a gelatinous band, always arranged in a more or less spiral 

 form, and fastened by one of its edges to corallines or the under sides of stones. 

 The ova are minute and very numerous, amounting in some species to several 

 thousands. Before the period of exclusion, the young may be seen revolving on 

 their own axis, by means of vibratile cilia, and on escaping from the egg they 

 swim about freely in the water by the same means. The larva is extremely minute, 

 and has more the appearance of a wheel animalcule than a mollusk. It is inclosed 

 in a transparent, calcareous, nautiloid shell, with an operculum. Its structure is 

 very simple, showing no signs of the external organs that distinguish the future 

 adult; the principal portion visible outside the shell being composed of two flat discs 

 or lobes, fringed with long cilia, by the motion of which it swims freely through 

 the water. These are often withdrawn into the shell, and the operculum is closed 

 upon them when the animal is at rest. Doris mabilla, a fine handsome species, having 

 a wide distribution in the Indian and Pacific Oceans is fully four inches long and two to 

 three in width. It has been obtained at the Seychelles, Andaman, and Samoa islands. 

 Bathydoris abyssorum was dredged in the mid-Pacific, at a depth of two thousand four 

 hundred and twenty-five fathoms. It is a large animal, about five inches in length, 

 of a nearly spherical form, subgelatinous, subpellucid, and greenish white, with a 

 dark purple foot. The branchiae are nonretractile, and disposed in six groups. It 

 forms a remarkable connecting link between the Tritoniidce and the Dorididce. In 

 the genus Hexabranchus the gills are arranged in a circle round the vent, and are 

 composed of six separate plumes, each of which is retractile within a special cavity 

 of its own, and not within a common cavity as in Doris. The species are not num- 

 erous, and have only been met with in warm seas, such as the Red Sea, and Indian 

 and Pacific Oceans. H. sandwichensis , a handsome species of a pale crimson tint 

 occurring at the Sandwich islands, is nearly six inches in length when alive. 



