10 This digestive cavity is really homologous to the proboscis of the jellyfish, turned in. It is lined with ectoderm. The "body cavity" is not really such, but is homologous to the digestive sac of the hydra.

11 Among the exceptions are Tubipora, which have eight tentacles and no septa, and the extinct Cyathophylla, whose septa are eight or more.

12 The longer septa (called primary) are the older; the shorter, secondary ones are developed afterward. As a rule, sclerodermic corals are calcareous, and a section is starlike; the sclerobasic are horny and solid. The latter are higher in rank.

18 The most important genera are Terebratula, Rhynchonella, Discina, Lingula, Orthis, Spirifera, and Productus. The first four have representatives in existing seas. Most naturalists now admit their affinity to the worms, some still keep them in the branch Mollusca, while others include them in the separate branch Molluscoida.

14 Some starfishes (Solaster) have twelve rays. In all echinoderms, probably, sea water is freely admitted into the body cavity around the viscera.

15 The shell is not strictly external, like the crust of a lobster, but is covered by the external skin.

16 Six hundred pieces have been counted in the shell alone, and twelve hundred spines. The feet number about eighteen hundred. They can be protruded beyond the longest spines.

17 Certain crabs live on dry land, but they manage to keep their gills wet.

18 The student should remember that this threefold division is not equivalent to the like division of a vertebrate body.

19 Each ring (called somite) is divisible into two arcs, a dorsal and ventral.

20 The eye stalks were formerly considered to be appendages, but are no longer so regarded.

21 These parts do not correspond to the parts so named in human anatomy. See also pp. 371, 372.

22 The four pairs of legs in arachnids answer to the third pair of maxillæ and the three pairs of maxillipedes in the lobster. The great claws of scorpions and the pedipalpi of spiders correspond to the first maxillæ of the lobster.

23 Compare the single thread of the silkworm and other caterpillars.

24 The common spider, Epeira, which constructs with almost geometrical precision its net of spirals and radiating threads, will finish one in forty minutes, and just as regularly if confined in a perfectly dark place.

25 There are some exceptions: the oyster is unequivalved, and the pecten equilateral.

26 The chief impressions left on the shell are those made by the muscles -the dark spots called "eyes" by oystermen; the pallial line made by

is diversified, though comparatively sparse. Examples of all the five invertebrate divisions were found in the Bay of Biscay, at the depth of 2435 fathoms.179

Distribution in the sea is influenced by the temperature and composition of the water and the character of the bottom. The depth acts indirectly by modifying the temperature. Northern animals approach nearer to the equator in the sea than on the land, on account of cold currents. The heavy aquatic mammals, as whales, walruses, seals, and porpoises, are mainly polar.

The land consists of the following somewhat distinct areas the Neotropic, comprising South America, the West Indies, and most of Mexico; the Nearctic, including the rest of America; the Palearctic, composed of the eastern continent north of the Tropic of Cancer, and the Himalayas; the Ethiopian, or Africa south of the Tropic of Cancer; the Oriental, or India, the southern part of China, the Malay Peninsula, and the islands as far east as Java, Borneo, and the Philippine Islands; and the Australian, or the eastern half of the Malay Islands and Australia. These are the regions of Sclater and Wallace. Other writers unite the northern parts of both hemispheres into one region, and the Oriental with the Ethiopian regions.

Life in the polar regions is characterized by great uniformity, the species being few in number, though the number of individuals is immense. The same animals inhabit the arctic portions of the three continents; while the antarctic ends of the continents, Australia, Cape of Good Hope, and Cape Horn, exhibit strong contrasts. Those three continental peninsulas are, zoologically, separate worlds. In fact, the whole southern hemisphere is peculiar. Its fauna is antique. Australia possesses a strange mixture of the old and new.

South America, with newer mammals, has older reptiles; while Africa has a rich vertebrate life, with a striking uniformity in its distribution. Groups, old geologically and now nearly extinct, are apt to have a peculiar distribution; as the Edentata in South America, Africa, and India; the marsupials in Australia and America; the Ratitæ in South America, Africa, Australia, and New Zealand.

In the tropics, diversity is the law. Life is more varied and crowded than elsewhere, and attains its highest development.

The New World fauna is old-fashioned, and inferior in rank and size, compared with that of the eastern continents.

As a rule, the more isolated a region the greater the variety. Oceanic islands have comparatively few species, but a large proportion of endemic or peculiar forms. Batrachians are absent, and there are no indigenous terrestrial mammals. The productions are related to those of the nearest continent. When an island, as Britain, is separated from the mainland by a shallow channel, the mammalian life is the same on both sides.

Protozoans, cœlenterates, and echinoderms are limited to the waters, and nearly all are marine. Sponges are mostly obtained from the Grecian Archipelago and Bahamas, but species not commercially valuable abound in all seas. Coral reefs abound throughout the Indian Ocean and Polynesia, east coast of Africa, Red Sea, and Persian Gulf, West Indies, and around Florida; and corals which do not form reefs are much more widely distributed, being found as far north as Long Island Sound and England. Crinoids have been found, usually in deep sea, in very widely separated parts of the world-off the coast of Norway, Scotland, and Portugal, and near the East and West Indies. The

other echinoderms abound in almost every sea; the starfishes chiefly along the shore, the sea urchins in the Littoral zone, and the sea slugs around coral reefs. Worms are found in all parts of the world, in sea, fresh water, and earth. They are most plentiful in the muddy or sandy bottoms of shallow seas. Living brachiopods, though few in number, occur in tropical, temperate, and arctic seas, and from the shore to great depths. Polyzoa have both salt and fresh water forms, and annelids include land forms, as the earthworm and some leeches.

Mollusks have a world-wide distribution over land and sea. The land forms are restricted by climate and food, the marine by shallows or depths, by cold currents, by a sandy, gravelly, or muddy bottom. The bivalves are also found on every coast and in every climate, as well as in rivers and lakes, but do not flourish at the depth of much more than two hundred fathoms. The freshwater mussels are more numerous in the United States than in Europe, and west of the Alleghanies than east. The seashells along the Pacific coast of America are unlike those of the Atlantic, and are arranged in five distinct groups: Aleutian, Californian, Panamic, Peruvian, and Magellanic. On the Atlantic coast, Cape Cod and Cape Hatteras separate distinct provinces. Of land snails, Helix has an almost universal range, but is characteristic of North America, as Bulimus is of South America, and Achatina of Africa. The Old World and America have no species in common, except a few in the extreme north.

The limits of insects are determined by temperature and vegetation, by oceans and mountains. There is an insect fauna for each continent, and zone, and altitude. The insects near the snow line on the sides of mountains in the temperate region are similar to those in polar lands.

The insects on our Pacific slope resemble those