74 



COELENTERATA HYDROZOA 



Fam. Monobrachiidae. — Monohrachium, found in the White 

 Sea by ]\Iereschko\vsky, forms a creeping stolon on the shells of 

 TcIIiiii(. The zooids of the hydrosome have only one tentacle. 



Fam. Myriothelidae. — This family contains the single genus 

 Myriothda. The zooid of the hydrosome stage is solitary and is 

 provided, as in the Coryuidae, with numerous scattered capitate 

 tentacles. The gonophores are borne by blastostyles situated 

 above the region of the tentacles. In addition to these blastostyles 

 producing gonophores there are, in M. 2J^^'''yg'ici, supplementary 

 blastostyles which capture the eggs as they escape from the 

 gonophores and hold them until the time when the larva is ready 

 to escape. They were called " claspers " liy Allman. In some of 

 the Arctic species Frl. Bonnevie ^ has shown that they are absent. 

 Each zooid of ^^. jiltrmiia is hermaphrodite. 



Fam. Pelagohydridae. — This family was constituted by 

 Bendy " for the reception of Pclagohydra mirahilis, a remarkable 



new species discovered by 

 : ' him on the east coast of 



the South Island of New 

 Zealand. The hydrosome 

 is solitary and free- 

 swimming, the proximal 

 portion of the body being 

 modified to form a float, 

 the distal portion form- 

 ing a flexible proboscis 

 terminated by the mouth 

 and a group of scattered 

 manubrial tentacles. The 

 tentacles are filiform and 

 scattered over the surface 

 of the float. Medusae 

 are developed on stolons 

 between the tentacles of the float. They have tentacles arranged 

 in four radial groups of five each, at the margin of the umbrella. 

 As pointed out by Hartlaub,^ Pelafiohydra is not the only 

 genus iu which the hydrosome floats. Three species of the genus 

 Margelo2Jsis have been found that have pelagic habits, and two 



1 Zeitschr. f. wiss. Zool. Ixiii. 1898, p. 489. 

 - Quart. Journ. Micr. Sci. xlvi. 1902, p. 1. =* Zool. Zcntralhl. x. 1903, p. 27. 



Ten.Tl. 



Fig. \Zi.— Pclagohydra mirabilis. Fl, The float ; 

 J/, position of the nioutli ; Ten.Fl, filamentous 

 tentacles of the float. (After Dendy.) 



