THE VOYAGE OF H.M.S. CHALLENGER. ZOOLOGY. REPORT on the P t e r o p o d a collected by H.M.S. Challenger during the Years 1873-76. By P a u l P e l s e n e e r , D.Sc. (Brussels). PART III.—ANATOMY. INTRODUCTION. The collection of Pteropoda, preserved in alcohol, brought home by the Challenger, includes specimens of all the known genera exceptCymbuliopsis andClione. This collection having been placed in my hands for purposes of systematic study, I was, when this part of the work was concluded, in an exceptionally favourable position for undertaking a monographic examination of the organisation of this group of animals. Since the publication of Souleyeti memorable work, the anatomy of the Pteropoda has been studied almost exclusively by means of European species, and this is the first time since that epoch that such an important collection as the present one has been made, including, as it does, forty-four species out of the sixty-three actually known, and specimens of nearly all the genera. Hence although the time at my disposal was very limited, I could not allow an oppor­ tunity to escape which would probably not soon occur again. By adding to the Challenger collection some specimens ofClione limacina which Mr. John Murray placed at my disposal, and a specimen ofCymbuliopsis of my own, I was able to investigate all the known genera of Pteropods, so that the conclusions at which I have arrived have not been drawn from the organisation of a few species only, 2 THE VOYAGE OF H.M.S. CHALLENGER. but from that of the entire group, each genus, and usually several species of each genus, having been studied. As the conclusion of these researches an analytical exposition of the whole organisa­ tion of each genus might be expected, but this would have extended the present Report beyond reasonable dimensions, and would have occasioned much repetition, since several of the genera of Pteropods arc very closely related, and the number of distinct types is fai' from large. Besides, as I have already remarked, the limited time at my disposal did not permit me to enter upon an anatomical monograph, and indeed, even if a longer period had been available, the following considerations would .have deterred me from such a course :— 1. We already possess a very considerable acquaintance with numerous points in the organisation of the Pteropoda, thanks to the general works, based upon several different genera, of van Beneden,1 Souleyet,2 and Gegenbauri and to the special memoirs, treating only of a single form, of Eschricht4 and Wagner.6 In many respects an anatomical monograph would simply be a reprint of what has been published by these authors. 2. The systematic position of the Pteropods is the subject of much discussion, and their phylogenetic relationships have been very variously interpreted. For several years® I have followed Spcngel,7 along with Grobben8 and Boasi in the opinion, not shared by most zoologists, that the Pteropoda do not constitute a distinct class among the Mollusca, comparable with the Cephalopoda, Gastropoda, Scaphopoda, aud Pelecypoda. Further, I am, like Boas, of opinion that even within the Gastropoda they do not constitute a primary division, but only a group of much lower rank among the Opisthobranchiata division of the Tectibranchiata Eutliyneura. This opinion, it must be added, is not new ; it was expressed during the first half of this century by de Blainville.10 Souleyet11 was the first, and indeed the only, inves­ tigator who has attempted to give any proof of it, and he was not very successful, 1 Exercice« Zootomiques, Mém. Acad. Sei. Bruxelles, t xii., 1839 ; Mémoire sur la Limacina arctica, op. cit., t xiv.f 1841. 7 Voyage de la Bonite, Zoologie, t. ii. pp. 37-288, 1862. 9 Untersuchungen über Pteropoden und Heteropoden, 1855. * Anatomische Untersuchungen über dio Cliona borealis, 1838 ; in Danish, Anatomiske Undersögclser over Clione borealis, K . dantk. Vidensk. Sclsk. A fhandl., 7 D eel, p. 327. 6 Dic Wirbello8on des weissen Meeres, Bd. L pp. 89-120, 1885. 0 Dic Gcruch8organe und das Nervensystem der Mollusken,Zeitschr. f. wiss. ZooL, Bd. xxxv. p. 373, 1880. 7 The ceplmlic appendages of tho Gynmosomatous Pteropoda,Quart. Jouru. Micr. Sei., N.S., vol. xxv. p. 606, 1885. Description d’un Nouveau genre de Ptöropode Gymnosoma,Bull. Sei. Dép. Nord, p. 220, 1880. Recherches sur le Système Nerveux des Ptéropodes, Arch, de Biol., t vii. p. 127, 1886. • Morphologische Studien über den Harn- und Geschlechtsapparaten sowie die Leibesböhlo der Cophalopoden, Arb. Zool. Inst. W ien, Bd. v. p. 246, 1884. 0 Spolia atlantica, Bidrag til Pteropodernes Morfologi og Systematik, &c.,K. dantk. Vidensk. Sclsk. Skriv., Raekkc 0, Bd. iv. p. 12. 10 Manuel de Malacologie, p. 480. u Voyage de la Bonite, Zoologie, t ii. REPORT ON THE PTEROPODA. 3 because he had opposed to him the dogmatic views of Cuvier, which were then all- powerful. Recently, however, the authors above mentioned have returned to an opinion closely resembling that of de Blainville ; but in order to place this upon a firm basis, further demonstration is still necessary. ; : On these grounds I have proposed to make the present Report a comparative anatomical study of these animals rather than a descriptive anatomical monograph, and shall attempt mainly to throw light- upon their systematic position ; and by the per­ mission of Mr. John Murray to conclude this third part of my Report upon the Pteropoda by an exposition of my views regarding their relations to the other groups of Mollusca. Having thus defined the object of the present work and the spirit in which it is conceived, I may say a few words regarding the method which has been followed in its elaboration and the manner in which I have divided it. The first portion of this Report bears upon the descriptive anatomy of the Pteropoda, taken genus by genus, treating first of the Thecosomata, then of the Gymnosomata. As I have indicated above, no attempt will here be made to discuss the whole organisation of each genus, but I shall study especially— 1. Tlie points neglected or misinterpreted by previous authors, in order, if possible, to elucidate them. 2. The points which seem to me to have an important bearing upon the relations and systematic position of the Pteropoda. In this portion I shall not attack the question of the embryonic development of the Pteropoda. Too few embryos were collected by the Challenger to furnish any new facts which might serve as a basis for a discussion of this special subject. The histology of the Pteropoda, too, will only occupy an unimportant place in our discussion, for the specimens collected by the Challenger were not preserved with a view to histological examination. Hence, in order to fill up the numerous lacunæ which will occur in the present work, I hope to publish hereafter the results of my examination of fresh specimens at the Naples Zoological Station. In the second part it is proposed to study the relations and affinities of the group of Mollusca called Pteropoda by a comparison of their organisation (as ascertained by the investigation described in the first part) with that of other Mollusca. CONTENTS. PAOE Introduction , ............ 1 O rganisation o p t h e P teropoda , ......... 5 I. Thecosomata, ........... 6 A . F am ily I . Limacinidae, . * ß B. Family II. Cavoliniidæ, . .1 2 C. Family EU. Cymbuliidae ......... 21 D. Phylogenetic Relations of the Thecosomata among themselves, . 2 8 E. Summary on the Thecosomata, ........ 37 II. Gymnosomata, ........... 37 A. Family L Pneumonodermatidm, ....... 3 8 B. Family ü. Clionopsidæ,. 4 5 C. Family EU. Clionidae, ......... 47 D. Family IV. Halopsychidae ........ 51 E. Summary on the Gymnosomata, ........ 55 IIL Summary on the Pteropoda, ......... 56 A f fin it ie s and P hylogenetic R elationships o f t h e P teropoda , ..... 57 I. Historical, ........... 57 ü. Are the Pteropoda Cephalopoda! . .6 0 III. Are the Pteropoda Gastropoda ! ........ 75 IV. Special Affinities of the Thecosomata and Gymnosomata, . .7 7 A Thecosomata, .......... 79 B. Gymnosomata, .......... 8 2 V. Do the Pteropoda constitute a Primitive or Derived Group Î . 8 8 VI. Polyphyletic Origin of the Pteropoda, ........ 89 A. Origin of the Thecosomata, ........ 89 B. Origin of the Gymnosomata, ./...... 94 VEL Summary, . , . .96 V III. General Conclusions, .......... 95 I. ORGANISATION OF THE PTEROPODA. I propose to study the Thecosomata and the Gymnosomata separately, and as I have enumerated the differential characters of these two divisions in the Systematic Report on Gymnosomata,1 there is no need to repeat them here. The phylogenetic origin of the two groups will be examined in the sequel. I. THECOSOMATA. We shall now proceed to examine in succession each of the genera recognised in the Systematic Report on the Thecosomata.2 Fam ily I. Limacinidæ. The knowledge of the organisation of the Limacinidæ is of the utmost consequence in the morphological study of the Pteropoda, because the true phylogenetic relations of the typical Thecosomata (Cavoliniidæ) are entirely concealed by an adventitious modification upon which we shall enlarge hereafter. This masks their true relationship, and if the Limacinidæ did not exist at the present time it would probably be impossible to explain this modification, and consequently very difficult to establish the real affinities of the Thecosomata. The only information we possess regarding the organisation of the Limacinidæ is to be found in the memoir of van Beneden onLimacina arctica (= helicina),1 and in the rather brief anatomical study of the “ genus ” Sp irialis, published by Souleyet in the Voyage of the “ Bonite.”4 These two works, however, in addition to being brief, include numerous gaps and several inaccuracies, which defects I shall endeavour to remedy.