Development of Tentacles in Hydra. 251 



Halosaurns rostratus, 

 B. 9. D. 10. V. 9-10. L. transv. 13/6. 



The length of the head exceeds much the height of the 

 body. The snout very much produced, spatulate, its prajoral 

 portion being more than one half its length. Eye of moderate 

 size, its length being one third of the postocular portion of the 

 head, and considerably less, than the width of the interorbital 

 space. Maxillary scarcely reaching the front margin of the 

 eye. The length of the head equals its distance from the root 

 of the ventral, which is nearly entirely situated before the 

 dorsal. Nearly all the scales are lost : but some of the lateral 

 line remain ; they are much larger than the other scales ; and 

 on the tail, where the lateral line approaches the loAver profile, 

 these larger scales fill up all the space between the lateral line 

 and the anal fin. 



Mid Atlantic, 2750 fathoms. 



NemicJithys infans. 



Body much less elongate and eye much smaller than in 

 N. scolopacea. Vent twice as distant from the root of the 

 pectorals as is the latter from the eve. 



Mid Atlantic, 2500 fathoms. 



Cyema, g. n. Mur^enid. 



This genus is the type of a new group of Murtenidae allied 

 to the Nemichthyina. It combines the form of the snout of a 

 Nemichthys with the soft short body of a Leptocephalus] but 

 the gill-openings are very narrow and close together on the 

 abdominal surface. Vent in about the middle of the length 

 of the body ; vertical fin well developed, confined to and sur- 

 rounding the tail. Pectoral fins well developed. Eye very 

 small. 



Cyema atrum. 



The cleft of the mouth extends backwards to the end of the 

 head. Black. 



Pacific and Antarctic, 1500 and 1800 fathoms. 



XXIX. — On the Mode of Development of the Tentacles in the 

 Genus Hydra. By M. C. Mereschkowsky. 



[Plate Xn.] 



In my article on the new Hydroid Monohrachium parasitum* 



* Ann. k Mag. Nat. Hist. ser. 4, vol. xx. p. 220. 



17->^ 



