'^^^ ZOOGENESIS '^^'^ 



ceans (crabs, shrimps, lobsters, barnacles, sow-bugs, beach-fleas, 

 water-fleas, etc.), and various other types. The number of 

 included species — roughly six hundred thousand of which about 

 twenty thousand are crustaceans — is vastly greater than the 

 number included in all the other animal groups taken together. 

 But in spite of this the uniformity in the general structure is so 

 very great that the members of the group are always readily 

 recognizable, usually throughout their lives, or at least in the 

 adult or first stages. Arthropods are found everywhere on land 

 and in fresh vv^ater, and down to the greatest depths in the oceans. 

 Most of them are free-living, but very many are parasitic, usually 

 on other arthropods, but also on vertebrates and other creatures, 

 both on land and in the sea. Some of the so-called parasitic 

 barnacles can be recognized as arthropods only in the early stages, 

 the adults becoming almost or quite structureless. (Figs, i, 

 p. 5; 7-16, p. 2-1; 17-2.3, p. 33; 14-30, p. 47; 31, 32., p. 55; 64, 

 p. hi; 91-93, p. 175; 100, loi, p. 175; 12.9, 131-133, p. 103.) 



EcHiNODERMATA. — This large group includes the starfishes, 

 brittle-stars, sea-urchins, holothurians or sea-cucumbers, crinoids 

 or sea-lilies and feather-stars, and the extinct Mastoids and 

 cystideans. Echinoderms are found only in the sea, where they 

 are represented from between tide marks dow^n to the greatest 

 depths. They are especially characteristic of deep water. 

 (Figs. 6, p. 5; 35-38, p. 55; 41, 42., p. 71; 43, 44, p. 87; 88, 89, 

 109, p. 175; 12.0, p. 185.) 



Nemertea. — A moderate sized group including species which 

 are found mostly in the sea, though a few live in fresh water and 

 some on land. In the sea they live chiefly on the bottom, in mud 

 and especially under stones or other objects lying on mud. But 

 some live exposed, and a few others are free floating. Most of 

 them have the most extraordinary power of elongating and 

 contracting the body. Some break to pieces with the greatest 

 facility, each piece later developing into a complete individual. 

 They are all carnivorous. (Fig. 106, p. 175, young.) 



Ch^tognatha — arrow-worms. — A small group living wholly in 

 the sea, found everywhere in shallow or moderately deep w^atcr. 

 They are transparent in life, appearing like small thin glassy 

 fishes. They live freely suspended in the water. (Fig. 62., p. 11 1.) 



Priapuloidea. — A very small group wholly confined to the 

 sea, wherein it is widely distributed. 



