32 THE VOYAGE OF H.M.S. CHALLENGER. 



muscles. Tlie longitudinal muscular liands are in close relation with the proboscis-sac. 

 Their exact disposition was not made out, and their arrangement, as shown in the figure, 

 will possibly need correction. 



" On the whole, Pelagonemertes is a form of considerable zoological importance. In 

 the flattened form of its body, and in its dendrocoelous digestive tract the animal 

 resembles Planarians. Amongst the Rhabdocoeles the Prostomcw possess an exsertile 

 proboscis like that of Nemertines, but such an organ is present in no Dendrocoele. In 

 all particulars — in being unisexual, in the simplicity of the generative organs, in the form 

 of the nervous and vascular systems and of the proboscis, in the position of the mouth and 

 presence of an anus — in all essential structures Pelagonemertes is most distinctively a 

 Nemertine. Only in its remarkable dendrocoele 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 whicli is characteristic of so many otherwise 

 most widely differing pelagic animals. 



"The development of the dendrocoele intestine is very remarkable, in that the lateral 

 ramifications are apparently to be regarded as a series of buds occurring successively 

 from before backwaixls from a previously straight digestive tract, such as exists in other 

 Nemertines. In this the digestive tract difi'ers entirely from that of dendrocoelous 

 Planarians, such as Leiitoplana tremellaris, in which, as we know from the observations 

 of Keferstein (' Beitriige zur Anatomic und Entwickelungsgeschichte 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 yelkballs arrange them- 

 selves 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 Ceplialothrix, but 

 the animal must evidently be placed in a new family of Nemertines, for which I propose 

 the term Pelagonemertidse, thus characterised : — 



" Animal pelagic in habit. Body gelatinous, hyaline, broad and flattened. Proboscis 

 unarmed. Ciliated sacs absent. Special sense-organs absent. Digestive tract dendro- 

 coelous. 



"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 forms. 

 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 inhaljitant of the sea-bottom. 



