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



National Museum ; Professor Pagenstecher and Dr. George Pfeffer, by whose kindness I 

 was enabled to study those in the Hamburg Museum ; and the Zoological Commission 

 of the " Vettor Pisani," who sent to me for examination the Pteropods collected by 

 this ship. 



I have also to thank, for information and other assistance, Dr. John D. M'Donald, 

 R.N., Dr. J. E. V. Boas of Copenhagen, 1 Dr. W. Giesbrecht of Naples, and Professor 

 Ed. Perrier of Paris. In short, my best thanks are due to all those who, in any 

 way, have assisted me. 



I have described with much care all the species that I have been able to study, and I 

 have figured them whenever there was not a sufficiently exact representation of them in 

 existence. 



But in spite of all the material employed for the elaboration of this Report, it is impos- 

 sible to give here a complete monograph of all the living Gymnosomata. For, besides 

 the forms which are now well known, there are many others which are insufficiently 

 described by the naturalists who made them known, of which the types have been lost, 

 and which have not again been found since their discovery. Thus, we have only a few 

 indications of the existence of some species still nearly unknown. I have drawn attention 

 to these forms, hoping that it may facilitate their recognition by future explorers. 



After the description of the different species I shall examine : — 



1. Their geographical distribution. 



2. Their mutual phylogenetic relations. 



2. The Gymnosomata and their Distinction from the Thecosomata. 



The adult Gymnosomata are chiefly characterised as follows : — By the absence of a 

 mantle-skirt, pallia! cavity and shell ; by the presence of a well-developed head, bearing 

 two pairs of tentacles, of which the two posterior bear rudimentary eyes ; by two fins 

 of which the anterior edges axe not joined together backwards, above the mouth ; and by 

 the anus being situated at the right side of the body. 



Every Pteropod possessing these characters belongs to the Gymnosomatous group, 

 and every Pteropod which does not possesses these characters is a Thecosomatous 

 form, so clearly and well-marked is the separation between these two divisions. As I 

 before said, the anatomical characters of the Pteropoda entirely agree with their division 

 into Gymnosomata and Thecosomata. If these names are not strictly exact (because 



1 The text and the plates of this Report were already far advanced when Boas' work appeared (Spolia 

 atlantica. Bidrag til Pteropodernes Morphologi og Systematik, K. dansk. Vidensh Selsh Skriv., 1886). As Boas 

 and I are agreed on many points relative to the Gymnosomata, it must he said that our results have been quite 

 ndependently obtained. But I have taken advantage of Boas' special discoveries to render this monograph more 

 complete. 



