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BISCAYAN PLANKTON. 



Part III.— THE CHyETOGNATHA. 



By G. Herbert Powler, B.A., Ph.D., F.L.S., F.Z.S. 



(Plates 4-7.) 



Read 2nd June, 1904. 



^\cl7\^ 



Contents. 

 Pase 



Paae 



The Systematic Determination of Chsetognatlia .... 55 



Examination of the ' Research ' Specimens 58 



Sac/itta serratudentata, Krohn 58 



Sayitta furcata, Steinhaus 63 



Sagiita macrocephala, sp. n 65 



Sayitta zetesios, sp. n 67 



Sagitta hiimnctata^, Quoy et Gaimard 69 



ScKjitta decijnens, sp. n 70 



Sagitta sp. indet. ? juy 71 



Sagitta 1 planctonis, Steinhaus 71 



Sagittaf spp. indet. of hexapterine type 73 



Krohnia hamata, Moebius 7-1 



Krohnia hamata, !^[oebius, var 77 



Krohnia subtilis, Grassi 78 



General Remarks on the Vertical Distribution .... 79 



Summary of the Observed Distribution 82 



Table of the Occurrence of the commoner Forms .... 84 



Appendix : Chaetognatha from the Falkland Islands . 85 



Literature 85 



Explanation of the Plates 86 



On the Systematic Determination of Chjetognatha. 

 Considering the comparatively small number of species, few animals can be more 

 troublesome to identify than a large collection of Cha3tognatha. This results not only 

 from the inadequate diagnoses by many of the earlier describers, Imt also from the 

 scarcity and variability of the external diagnostic characters, the considerable differences 

 in the proportions to one another of the A-arious parts (jiroduced in the less firm species 

 by varying degrees of exjiansion and contraction), and the difficulties of satisfactory 

 preservation. 



Taking these points in a little moi'e detail — Oscar Hertwig reduced to order such 

 species as came under his notice, but unfortunately these were not numerous. Had all 

 later observers given equally careful descriptions, our task to-day would have been 

 easier ; but, as it is, cursory descriptions of a few external features, taken apparently, in 

 many cases, from only one or two sjiecimens, are the rule in the literatui'e of the 

 subject. Almost moi-e trouble has been caused by inadequate drawings * ; considering 

 how closely alike at first sight are even sucli undoubtedly " good " species as hexaptera 

 and enfiata, no freehand drawing of the outline of a Sagitta can be trusted, because a 

 quite small error in the proportions of head, body, tail, and fins may easily give the 

 misleading outline of another species. Every reporter on a collection of Chtetognatha, 

 who desires to help in placing the genera and species on a proper footing, should give an 



* Perhaps this sweeping statement may be held to require justification. Among recent papers, one author gives 

 measurements showing the tail-segment of a species to be 24 per cent., or nearly a quarter, of the total length, but 

 draws it 31 per cent., or nearly a third. Another writer says that " the posterior fins touch the vesiculte seminales," 

 yet draws them far apart : in another species he describes the tail-segment as a fourth of the whole length, and 

 draws it as 35 per cent., or more than a third. When one finds such inaccuracies in easily measurable matters, 

 absolutely no reliance can be placed on the figure as a whole. 



SECOND series. — ZOOLOGY, VOL. X. 10 



