The minute jellyfish stage, Lhzia blondina, of a hy- 

 droid colony. (England. D. P. Wilson) 



come out in only one plane, so that they resemble 

 every kind of plumage from small feathers to long 

 plumes, some growing up to 6 feet in very deep wa- 

 ters. The ostrich-plume hydroid. Aglaoplienia. which 

 looks like stout brown feathers, often about 3 inches 

 long, is especially common, after storms have washed 

 it loose, in the beach drift on the American Pacific 

 coast. Other species are known from the Atlantic 

 coast and European and many other shores as well. 

 The podlike structures conspicuous among the 

 branches are protective flaps covering the reproduc- 

 tive polyps. 



In the naked hydroids. the horny covering is ab- 

 sent or scanty or extends only as tubes that stop at 

 the bases of the feeding and reproductive polyps. 

 These include a number of common genera that 

 grow as rather large single polyps or as clumps or 

 mats of vertical colonial polyps; they are mostly pink 

 or rosy in color. Corymorphii is a solitary polyp up 

 to 4 inches long that lives in mud flats. It is related to 

 the solitary giant polyp. BranchioceiiuiUlnis. men- 

 tioned earlier as a deep-water form. CIciva and Tii- 

 hulariu grow in long-stemmed clumps ( Plates 4 and 

 5). Hydraainia, sometimes known as "hedgehog hy- 

 droid" because of the spininess of the encrusting mat 

 formed by the basal stalks that unite the colony, is 

 noted for the diversity of its several kinds of polyps, 

 and also for its habit of encrusting shells adopted by 

 hermit crabs. Whether it actually shares the host's 

 food we do not really know; in any case it can live 

 on rocks and wharf pilings, quite independent of the 

 crabs. 



Related to Clciva and the other naked types is 

 CordyUiphora luciisuis. the fresh-water colonial hy- 

 droid. In brackish inlets on marine coasts it is seen 

 as white, bushy, treelike colonies, a few inches high, 

 on stones, eelgrass, or wharf piles. When found in 

 rivers and lakes it seldom exceeds 1 inch in height. 



72] 



Of the medusas allied to the hydroids, few are as 

 big or as cosmopolitan as Aeqiioreii. or better known 

 from their summer swarms. White or pale bluish 

 green and somewhat flattened like the other medusas 

 of sheathed hydroids, it has many radially spreading 

 digestive branches, in some species more than a hun- 

 dred. The tentacles are a short fringe when con- 

 tracted, a long curtain of slender strands when fully 

 extended. The polyp is minute and not known for all 

 species. Stranded aequoreas, masses of jelly often 

 about as big as one's hand, sometimes much bigger, 

 are as well known to Japanese beachcombers as to 

 those of American or European coasts. The lumines- 

 cence for which they are noted can be rubbed off on 

 the hands as luminescent grains, then dabbed on the 



faces and hands of unsuspecting friends, creating 

 weird effects in darkness. It may be added that all the 

 hydroids mentioned earlier are also luminescent, 

 though not so bright a%Aequorea. 



Medusas allied to the naked hydroids are usually 

 deep-bodied little bells, like SarsUi, Lizzia, and Po- 

 doioryne. Many have never been matched to their 

 polyps. The need of the larva to attach to some kind 

 of solid substrate in those forms that have a fixed 

 polyp stage keeps most hydrozoan jellyfishes tied to 

 shore waters. Bigelow once observed that in the deep 

 and open waters ten miles off Bermuda only 3 per 

 cent of the jellyfishes he collected were of species 

 with a fixed stage in the life cycle. 



Hydras are naked little fresh-water polyps that 



abound in unpolluted streams, ponds, or the shore 

 waters of lakes, living attached to submerged stones, 

 twigs, vegetation, or debris. Yet they rarely come to 

 the attention of anyone who does not deliberately 

 seek them out, for when disturbed they contract into 

 all but invisible little knobs. Even when fully ex- 

 tended the cylindrical body is as fine as sewing 

 thread, only Vi of an inch long or so, and has gos- 

 samer tentacles that barely match the body in length, 

 or in some species are three or four times as long. 

 Their regular occurrence in certain favored spots 

 makes them the only living coelenterates readily and 

 dependably accessible at great distances from the 

 ocean. Though to the naked eye the hydra may ap- 

 pear as an insignificant little creature, under a hand 

 lens or a microscope it becomes a spectacular beast 

 that fully deserves to be named after the mythical 

 monster slain by Hercules. Hydra had nine heads, 

 and when Hercules cut one off. two grew in its place. 

 Hydra's small namesakes are renowned for their 

 ability to replace injured tentacles, grow a new 

 "head" if beheaded, grow two heads on one body, or 

 reproduce asexually by budding off new individuals 

 from the sides of the body. In sexual reproduction 

 the egg develops into a little embryo that may lie 

 dormant during the winter but in spring develops di- 

 rectly into another polyp without any kind of free- 

 swimming stage. 



The green hydra, Chlorohydra viridissima. which 

 harbors algal cells of the genus Chlorella. and the 

 brown hydra, Pebnalohydra oligaclis. in which the 

 slender basal stalk is more obviously set off from the 

 main part of the body than in other hydras, are cos- 

 mopolitan species, found on many continents. All 

 the other species are limited to particular continents 

 or continental regions. When green coloring occurs 

 in Pelmatohydru oligactis it is always due to eating 

 green food, such as green chironomid larvae. Starved 

 animals lose their color, while well-fed ones on the 

 usual diet are brownish or even black. Hydra circiim- 

 cincla is a lovely orange-red if fed on ostracods or red 

 copepods, and a rich color advertises the prosperity 

 of its bearer. 



Most large libraries have the original Memoire by 

 Trembley on the behavior and regeneration of hy- 

 dras. To read about his long romance with the little 

 "polypes d'eau douce en forme de cornes" is to want 

 to see the creatures feed, bud, and regenerate. For 

 readers who prefer English, John Baker's book, also 



Left. A hydra opens its mouth wide to swallow a wa- 

 ter flea. The minute crustacean victim is already para- 

 lyzed by the stinging threads. Rif^hl. The daphnid has 

 been swallowed and is in the hydra's digestive cavity; 

 the body outline and the large eye can be seen through 

 the body wall, ( Eric Grave ) 



173 



