ANTHOMEDUS.E MARGELOI'SIS. 



SI 



Medusa. Bell pyriform with a flat apex 1.3 times as high as wide. 2 mm. high. Gelat- 

 inous walls quite thick. 3 or 4 tentacles upon each of the 4 marginal tentacle-bulbs. These 

 tentacles are ringed with nematocysts. No ocelli. 4 narrow, straight radial-canals and a 

 wide axial-canal above the stomach. Manubnum wide, spindle-shaped, two-thirds as lout; 

 as the depth of the bell-cavity. The upper third ot the manubnum has no gonad, but the 

 lower part is incased in a tubular genital organ which bears planulae. Tentacle-bulbs 

 brown. Manubrium with dark-brown pigment granules. Found at Helgoland, German 

 Ocean, in July and early in August. 



Hydroid. The planulae which develop upon the manubrium of the medusa grow into 

 actinula larvae before being set free. The oral pole of the actinula is blunt and is adjacent 

 to the wall of the stomach of the medusa, while the aboral pole projects outward from the 

 stomach-wall into the bell-cavity. This aboral pole is conical and terminates in a sucker- 

 shaped depression. There is a row of 5 to 7 short, somewhat club-shaped tentacles around 

 the oral pole and two closely-set rows of alternately arranged tentacles near the aboral pole. 

 Both aboral rows have together about 12 tentacles, and these are somewhat longer than the 

 5 or 7 oral tentacles. The middle part of the body of the actinula is devoid of tentacles. 



FIG. 38. Hydroid and medusa of Margelopsis harckelii, after Hartlaub, in Nor.iisdu-s Plankton. 

 Medusa is about to set free an actinula larva. 



Medusa-buds develop after the actinula has been set free and has become about I mm. 

 long. These medusa-buds develop in clusters from the sides of the body of the actinula close 

 to the bases of the lower row of aboral tentacles with which they alternate in position. The 

 medusa-buds develop 2 or 3 tentacles in each of their 4 marginal clusters while still attached 

 to the actinula. Hartlaub found large numbers of these interesting hydroids floating in the 

 water at Helgoland, late in August. 



Miiller, 1908, has studied the origin and structure of the ova in the medusa. In common 

 with HvbocoJon,tYie eggs are large, amoeboid, and few in number when full grown, for the 

 successful eggs devour the weaker ones in the ovary. The ooplasma is composed of wide 

 irregular fibers, forming a somewhat narrow-meshed network. The exoplasma is distinct 

 from the endoplasma and is wider in its meshes. 



Margelopsis stylostoma Hartlaub. 



Margelofiis slylostoma, HARTLAUB, 1903, Zoolog. Centralblatt, Bd. 10, p. 2S, fig. 2, fig. 4 ( ?)i 1904, Wisscn. Mccrvsuntrrsuch. 

 Kommis. Deutsch. Meere, Abth. Helgoland, None Folge, Bd. 5, p. 99, fig. I, fig. z (?); I97> Nordisches Plankton, Nr. 

 i*, p. 91, fig. 87 (88?). 



This is a small spindle-shaped, free-floating hydranth, with a sucker-like expansion at 

 its aboral pole, and with three verticils of tentacles, i. e., 4 short-knobbed oral tentacles, a 



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