GASTROPODS. 355 



and all the beautiful and varied colours 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 escapino- 

 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-animacule than 

 a mollusc. It is enclosed 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 These are often withdrawn into the 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 Ocean, is fully 4 inches long and 2 to 3 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 5 inches in length, of a nearly spherical form, 

 subgelatinous, subpellucid, and greenish white, with a dark purple foot. The 

 branchiae are non-retractile, and disposed in six groups. It forms a remarkable con- 

 necting link between the Tritoniidce and the Dorididce. In the genus Hexa- 

 branchus 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 numerous, 

 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 6 inches in length when alive. 



The family Polyceridai is distinguished from the Dorididce by having non- 

 retractile gills; the principal genera being Goniodoris, Acanthodoris, Idtdin. 

 Ancula, Polycera, Plocamophorus, Triopa, and AUgirus. Ancula cristata is an 

 elegant little creature, about half an inch in length, occurring upon most of the 

 British coasts. It is white, with the processes tipped with yellow or orange. 

 The tentacles are laminated and non-retractile, each having two styliform 

 appendages at the base. The gills are placed in the middle of the back, on each 

 side of which there are a few compressed appendages. 



This division of Nudibranchs was established for a group of 

 naked marine molluscs having the gills placed symmetrically along 

 each side of the body between the margin of the dorsal mantle and the edge of the 

 creeping disc. Phyllidia and Pleiiropliyllidia are the typical genera originally 

 described, and may be regarded as the principal representatives of this group of 

 molluscs. One group of Inferobranchs, however, is abnormal in being destitute of 

 external branchiae. In the genus Phyllidia, containing several very handsome 

 species, the animal is somewhat depressed, and covered with a leathery and sometimes 

 tuberculated mantle; the head is small and concealed between the foot and back : 

 and the two oral tentacles are short, the dorsal pair retractile into cavities towards 



