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



Dexiobranchasa, 1 Boas. 



1855. Pneumodermon (j>ars), Gegenbaur, Untersuchungen iiber Pteropoden und Heteropoden, 



p. 74. 

 18G2. Pneumodermopsis, Bronn, Die Klassen und Ordmmgen des Thierreichs, Bd. iii. pp. 584, 645. 

 1885. Dexiobranchxa, Boas, Vorlaufige Mittheilung iiber einige Gymnosomen Pteropoden, Zool. 

 Anzeiger, No. 210, p. 688. 



Body rather long, pointed posteriorly. Visceral mass not extending to the posterior 

 end of the body. 



Head moderately lengthened ; anterior (labial or oral) tentacles strong. 



Foot ; posterior lobe pointed at its free extremity. 



Lateral Gill; a simple, more or less long appendage, of a triangular shape, without 

 longitudinal crests or fringes. 



Proboscis moderately long. The radilla in the adult has always a median tooth. 

 The hook-sacs are but little developed, and consist of small deep depressions which con- 

 tain short hooks. 



Buccal Appendages. — There are morphologically three acetabuliferous appendages ; 

 one median, which becomes anterior during the evagination of the proboscis, and always 

 bears five suckers, and two lateral appendages. The two latter, or the median appendage 

 alone, or all three appendages, may be atrophied, and their suckers are then directly 

 inserted on the ventral wall of the buccal cavity, forming groups corresponding to the 

 atrophied appendages. The five suckers of the median appendage, or of the corresponding- 

 group, are always disposed in the following manner — a median distal sucker on the free 

 extremity of the appendage, and two lateral ones on each side. The number of the 

 suckers of the lateral appendage varies according to the species. 



The skin is pigmented, and the two posterior ciliated rings generally remain a very 

 long time. 



Boas chiefly contributed to make this genus 2 well known ; he was the first to 

 describe with precision the acetabuliferous appendages, and of five species now known he 

 discovered three. I think the name Dexiobranchsea, that he gave to this group, ought to 

 be preserved, although Bronn had previously formed the genus Pneumodermopsis for the 

 same type from one of its principal characters (viz., the absence of a posterior gill), 

 because the latter genus was founded on the insufficient description, without a figure, 

 which Gegenbaur gave of Pneumodermon ciliatum. But Boas was the first to make 

 known the true characters of this genus, and he has figured specimens under the name 

 of Dexiobranckwa, and it is therefore desirable to preserve this last name, rather than 

 that of Pneumodermopsis, which accords with no good characters nor figures. 



As I have said, this genus includes five species, that may be distinguished as follows : — 



1 From h%ios, on the right side, and li^ayxia, gill. 



2 Von Ihering regarded it as a larva (Vergleichende Anatomie des Nervensystems und Phylogenie der Mollusken 

 p. 24G). 



