LEPTOMEDUS.E — OBEUA. 239 



brought to light; yet some of these differences in the hydroids and young medusae are not of 

 specific value, but are probably due to the influence of environment upon the individual, 

 such as favorable or unfavorable conditions of growth, geographical situation, depth and 

 temperature of water, light, etc. It is also true that some of the observed differences in the 

 hydroid stocks are probably of good specific value, and that we have in Obelia a genus in 

 which the mature medusae of the various species closely resemble one another, while fairly 

 well-defined differences exist in the hydroids and young medusae. It is quite true, however, 

 that the young medusae of one and the same species, at the time of liberation from the repro- 

 ductive calycles, vary considerably both in the number of tentacles which they possess and in 

 the state of development of the gonads. The study of every fairly large collection of these 

 hydroids seems to have led only to the burdensome addition of "new" species founded upon 

 minute distinctions. 



Undoubtedly many "species" have been founded on mere geographical or environmental 

 characters of a racial rather than specific nature, and hopeless confusion has long since been 

 introduced into the synonymy. The following description of the so-called species is presented 

 mainly with the hope that an inspection of the inextricable confusion here displayed may deter 

 future students from further inflictions of such "discoveries" upon the world. Every bushel 

 basket full of Obelia hydroids, collected at random along our shore, is sure to contain sev- 

 eral dozens of "new species." 



Species of Obcha are widely distributed along the shores of the temperate oceans, but they 

 are rare in tropical regions. The rich growth of kelps and fucus along the rocky shores of 

 temperate regions affords abundant anchorage for the hydroids of Obelia, but in the tropics 

 large seaweeds are absent in shallow water, and this fact may account for the rarity of Obelia 

 in warm oceans. Many of the species are of very wide distribution, being found in the Atlantic 

 and the Pacific and in the northern as well as the southern hemispheres. They are all lit- 

 toral forms, and the medusae are not found far from coasts. 



De Varenne, 1883, and Weismann, 1883, find that the sex-cells of Obelia originate in the 

 entoderm of the stem of the hydroid and later migrate into the medusa-bud, pushing through 

 the entoderm into the ectoderm, which finally incases them. Interesting studies of the wan- 

 derings of the developing sex-cells in the medusa have been conducted by Hartlaub and 

 are referred to in detail in the description of O. adelungi and O. helgolandica. 



The macroscopic details of development of Obelia have long been known. In 1883, 

 Mereschkowsky found that the entoderm arises from cells which wander singly into the interior 

 from the posterior end of the pear-shaped planula larva. (See Bull. Soc. Zool. de France, 

 tome 1883, 8, p. 98.) 



Regeneration in Obelia has been studied by Davenport, 1894 (Anatomischer Anzeiger, 

 Bd. 9, p. 283). He finds that the regenerative tissue is not differentiated at different places to 

 produce different things, independent of environment; but on the contrary, the embryonic 

 tissue at all levels may produce the same things. Moreover, there is no important indication 

 of difference in the germ-plasma at different levels in respect to the size or form of the parts 

 which regenerate. The hydroids display a marked tendency to regenerate 3 to 6 or 9 to 1 1 

 rings when cut at any point on the stem. 



These results of Davenport's should, however, be interpreted in connection with the 

 extensive work of Morgan, 1905 (Journal Experimental Zoology, vol. 2, p. 495, and ibid., 1906, 

 vol. 3, p. 501). Morgan finds that Tubularia displays a definitely recognizable although not 

 absolute polarity. A stolon is very rarely, and a hydranth commonly, regenerated upon the stem 

 it it is cut across near the terminal hydranth, whereas a stolon is often regenerated from the 

 aboral end of the stem if cut across near the lower end of the stem. The aboral end of a cut-off 

 piece of stem may, however, be caused to regenerate a hydranth if the oral end, near the base 

 of the polypite, be tied with a ligature, as was first shown by Loeb. Loeb concluded that the 

 polarity in the whole stem was reversed by this process of tying a ligature, but Morgan refutes 

 this idea and shows that the effect is confined to the aboral end itself which is regenerating 

 a polypite. Morgan concludes "that a gradation of hydranth-forming substance is present in 

 the stem of Tubularia, and that the amount present at any level determines the rate at which 

 both the oral and basal hydranth develop." He also concludes that "the farther the level of 

 the stem from the hydranth the greater its differentiation as stem; hence its gradation of 



