GA S TR O PODS, 
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 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-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 These are often withdrawn into the 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. Bathycloris 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 Dorididcu. 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 Dolyceridce is distinguished from the Dorididce by having non- 
retractile gills; the principal genera being Goniodoris, Acanthodoris, Idalia, 
Ancula, Polycera, Plocamophorus, Triopa, and AEyirus. 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 Pleurophyllidia 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 
Inferobranchiata. 
