ABSTRACT
The minute creature to which the above name has been given, though abundant, perhaps, in all seas, and noticed so long ago as in the year 1781, has received but little attention from zoologists in general. Its curious and interesting structure, however, and doubtful position in the animal kingdom, render it a subject well worthy of further research ; and its minute size, and the extreme delicacy and transparence of its tissues, make it peculiarly an object of microscopical investigation. Though perhaps unknown, even by sight, to many of our readers, the Sagitta bipunctata will probably be met with on every part of the coast; and it may be procured, without difficulty, at any rate in fine and calm weather, by means of a small muslin towing net over the side of a boat.
’ Physikalische Belustigungen, oder mikroskopische Wahrnebmungen von 43 in-und auslandischen Wasser-und Landthierchen.’ Nurnberg, 1781
’ Account of the Arctic Regions,’ vol. ii., Plate XVI.
“Voyage dans l’Amerique meridionale; Mollusques, p. 140, Plate X., figs. 1”7
’ Annals Nat. Hist.,11843
| ‘Annals Nat. Hist.,’ 1st Ser., vol. xiii., p. 1.
’ Anatomisch-physiologischeJBeobachtungen ub. die Sagitta bipimcata,’1844. ‘Nachtragliche Bemerkungen ub. den Bau der Gattung Sagitta, nebst der Bescbreibnng einiger neuen Arten.’ (Wiegmann’s 1 Archiv.,’ 1833, p. 266, Plate XII.) And Muller’s ‘Archiv.,’ 1853, p. 140.
* ‘Observations de Sagitta, mare Germanicum circa insulam Helgoland incolente,’ 1846.
† Report of British Association, 1851. (Trans, of Sections, p. 77.)
Beobachtungen lib. d. Anatom. u. Entwicklung einiger wirbellos. Seethiere, 1851, p. 93.
L. c, p. 11, Fig. 1,16;
Krohn remarks (p. 13), that a similar movement of the spermatic globules is observed in the testicular vesicles of the Leech.
In Siebold and Kölliker’s ‘Zeitscbrift f. Wissens. Zoologie,’ Bd. v., p. 15, is a short notice respecting the development of Sagitta, by C. Gegenbaur. He states, that the process of segmentation terminates in the production of an embryo of a rounded form, in which two kinds of cell-masses may be recognized,—one central, constituted of minute, and a well-defined peripheral layer, composed of larger cells. A depression is now formed at one point of the surface, which gradually advances to the centre, constituting the rudiment of the intestine. The embryo now appears to increase in length, in consequence of which, since it completely fills the cavity of the ovum, it becomes bent, and is ultimately coiled in a vermiform fashion. The cavity of the trunk may be distinguished, traversed by the intestine, which forms, as it were, a vertical septum ; but, besides this, no other internal organs are apparent. At this period the embryo often moves, and on the addition of acetic acid the muscular bands in the trunk are visible, completely formed, and exhibiting the fine transverse striæ. The fins arise as simple lateral outgrowths of the body. In this condition the animal leaves the ovum, about J’” in length, and already presenting in all respects the character of the full-grown Sagitta. The other organs, consequently, are not developed until after the animal has quitted the ovum. In the entire course of development, many stages of which, particularly those which succeed complete segmentation, are very difficult to be understood, cilia never make their appearance.
If the anatomy of this creature had not satisfactorily shown that it belongs neither to the Pteropoda, nor to the Heteropoda—this would have been rendered certain by its mode of development, which does not accord in any respect with the Molluscan type. What the real position of Sagitta is I will not determine.
