Specimen of Pelagonemertes Rollestoni. 381 



the mouth and presence of an anus — in all essential structures 

 Pelagonemertes is most distinctively a Nemertine. Only in its 

 remarkable dendroccele intestine does it differ from all other 

 Nemertines, and (but this is of far less importance) in the 

 modification of its tissue into the peculiar hyaline gelatinous 

 condition which is characteristic of so many otherwise most 

 widely differing pelagic animals. 



The development of the dendroccele intestine is very re- 

 markable, in that the lateral ramifications are apparently to be 

 regarded as a series of buds occurring successively from before 

 backwards from a previously straight digestive tract such as 

 exists in other Nemertines. In this the digestive tract differs 

 entirely from that of dendrocoelous Planarians, such as Lepto- 

 plana tremellwis, in which, as we know from the observations 

 of Keferstein (" Beitrage zur Anatomie und Entwickelungs- 

 geschichte einiger Seeplanarien von St. Malo/' Abhandl. der 

 k. Gesellschaft der Wiss. zu Gottingen, 4ter Band, Gottingen, 

 1868, Taf. iii. figs. 19, 20 ; 21, text p. 34), " the great yelk- 

 balls arrange themselves in the embryo with regularity and 

 map out the form of the future digestive tract," the peripheral 

 ramified part of the tract being formed at the same time as the 

 central portion. 



The peculiar form of the front of the body of Pelagonemertes 

 may be regarded as an instance of the excessive formation of 

 the head-lappets of many Nemertines. In having no ciliated 

 sacs and an unarmed proboscis, Pelagonemertes resembles 

 Cephalotkrix ; but the animal must evidently be placed in a 

 new family of Nemertines, for which I propose the term 

 Pelagonemertidas, thus characterized : — 



Pelagonemertidae, fam. nov. H. N. M. 



Animal pelagic in habit. Body gelatinous, hyaline, broad 

 and flattened. Proboscis unarmed. Ciliated sacs absent. 

 Special sense-organs absent. Digestive tract dendrocoelous. 



The occurrence of a second specimen of Pelagonemertes off 

 Japan shows that the animal has a wide distribution. It was 

 found on both occasions adhering to the trawl-net, and is, from 

 its very slight consistency, easily overlooked. Hence it may 

 have been often missed by us, and probably is as widely 

 distributed as other oceanic foi*ms. Since it has never been 

 taken, by former observers of pelagic animals nor by us, in 

 the tow-net, it is very probable that it occurs only in deep 

 water, and does not come to the surface ; it is, however, most 

 evidently not an inhabitant of the sea-bottom. 



