ters and many anemones of temperate and cold 

 marine waters look green from the plantlike cells 

 that live in their tissues. But in warm shallow seas 

 such relationships are the rule rather than the excep- 

 tion. On coral reefs almost all coelenterates, jelly- 

 fishes included, harbor colored cells; and we believe 

 that this contributes to their astounding success in 

 warm waters. 



Reproduction is both sexual and abundandy asex- 

 ual. Like the sponges or any other group that readily 

 reproduces by asexual means, coelenterates have ex- 

 traordinary capacity to regenerate lost tentacles or 

 branches, to grow by budding, or to reproduce by 

 rupture of the body. Some species can regroup their 

 cells and grow again even after the tissues have been 

 dissociated experimentally by being passed through 

 fine cloth, thus giving us answers to many problems 

 dealing with the enviable adaptability of tissues less 

 specialized than our own. When both polyps and 

 jellyfishes appear in the life cycle, the reproductive 

 chores are divided between a polyp that reproduces 

 asexually by budding ofT medusas, and a medusa 

 that reproduces sexually by shedding eggs or sperms 

 into the water. The medusa may be free-swimming, 

 or it may never be set loose, remaining always at- 

 tached to the polyp or polyp colony. The fertilized 

 egg of marine coelenterates develops into a little oval 

 or elongate free-swimming larva, called a planula, ' 

 which swims about for a time, then setdes down, at- 

 taches to the bottom, and grows into a polyp or, 

 by budding, into a polyp colony. Many modifications 

 and short cuts occur in this typical scheme. 



Of the three classes of coelenterates, the first or 

 most primitive one is the Hydrozoa, in which many 

 species have both polyps and medusas, often equally 

 developed, in the life history. The Scyphozoa have 

 minimized or dropped the polyp, staking their for- 

 tunes on larger and better jellyfishes. The Anthozoa 



Feather hydroid, Kirchenpaiiciia (Phimidaria) pin- 

 nata. About 2 to 3 inches high. (England. D. P. 

 Wilson ) 



have gone off in the other direction and have given 

 up the medusa altogether, the sea anemones per- 

 fecting a larger and more muscular polyp, the corals 

 specializing in great skeletal works. 



Hyclrozoaii Polyps and 



Jellyfishes iClass Hydrozoa) 



Hydrozoans are named from the resemblance of 

 their polyps to the little solitary hydras of fresh wa- 

 ter, but members of this class may also have a me- 

 dusa stage. We are not always able to match the pol- 

 yps we find attached on the bottom to the properly 

 related little gelatinous medusas that swim and feed 

 at the surface and reproduce sexually. Often polyps 

 and medusas are separately described and named, 

 and their relationship as stages of a single life cycle 

 sometimes emerges only when we happen to see the 

 medusa released, by an asexual process, from a 

 polyp confined in an aquarium. 



This is an important group, with close to three 

 thousand species, and almost all are entirely marine; 

 but it does include those few little polyps and me- 

 dusas that live in fresh waters. The tentacled polyps 

 of hydrozoans differ from those of the other classes 

 in having a simple digestive cavity, undivided by 

 ridges or partitions. The hydrozoan medusa can usu- 

 ally be readily told from the true or scyphozoan jel- 

 lyfishes by its smaller size and by its possession of a 

 velum, a transparent and muscular circular shelf 

 that projects inward from the rim of the gelatinous 

 umbrella (or saucer, or deep bowl, or bell, or what- 

 ever name best fits the variously shaped medusas). 



Hydrozoans are notable in the animal kingdom 

 for the many shapes that members of a hydroid col- 

 ony assume. Aside from the division of reproductive 

 chores between polyps and medusas, the polyps 

 come in a variety of body forms, each specialized 

 for its share of the colony housekeeping. Tentacled 

 feeding polyps and club-shaped reproductive polyps 

 are a frequent combination, but some colonies have 

 one or more kinds of mouthless polyps that serve 

 only for stinging prey or for protection. In other spe- 

 cies there may be in addition a striking ditTerence 

 between the reproductive polyps that produce female 

 medusoid individuals and those that produce male 

 medusoids, both bearers of sex cells. 



THE HYDROIDS 



The hydroids are called "sea firs'" in England and 

 sometimes "sea plumes" in the United States. These 

 names indicate the branching patterns of some com- 

 mon colonial forms, but they are rather misleadingly 

 grand designations for most hydroids, which are 



