26 BUSK, ON SAGITTA BIPUNCTATA. 



anterior denticles on each side and 18 posterior. The 

 bundles of rigid setm are scattered irregularly over the surface. 

 The only individual met with by Krohn was one centimetre 

 in length. 



Other species described by authors are — 



5. S. cephaloptera (Busch, 1. c, pi. xv., fig. 2). — Distin- 

 guished by a radiated disc on the anterior part of the trunk, 

 and two tentacular processes on the sides of the head. 



6. S. rostrata (Busch, 1. c, fig. 7). — Distinguished from 

 S. setosa, Wilms, by the presence of a large rounded eminence 

 on the anterior part of the head, which Krohn imagines may 

 be caused in a young specimen of a Sagitta by the cephalic 

 ganglion. 



7. S. bipunctata, Quoy and Gaimard, which we regard as 

 identical with — 



8. S. setosa, Wilms, the species here described, and pro- 

 bably the parent of other species, among which perhaps may 

 be enumerated those named by D'Orbigny (Voyage dans 

 I'Amerique Meridionale, torn v., p. 14, PI. 10) according to 

 the number of their fins, as S. diptera, S. trijHera, and S. hex- 

 aptera. If all these really belong to Sagitta at all, which, 

 in the absence of farther information than that given by 

 D'Orbigny, may be regarded as doubtful, S. hexaptera, at 

 any rate, may be considered identical with S. hipunctata. 



With respect to the systematic position of Sagitta, very 

 considerable difficulties arise in the determination of it. Mr. 

 Huxley (1. c, p. 77) remarks that ^'■Sagitta has been placed 

 by some among the Mollusca, a view based upou certain 

 apparent resemblances with the Heteropoda. These, however, 

 are superficial ; the buccal armature of Sagitta, for instance, 

 is a widely-different structure from the tongue of JFirola to 

 which, when extended, it may have a distant resemblance." 

 " There appears," he says, " much more reason for placing 

 this creature, as Krohn, Grube, and others have done, upon 

 the annulose side of the animal kingdom ; but it is very 

 difficult to say in what division of that sub-kingdom it may 

 most naturally be arranged." After surveying the points of 

 resemblance and difference between Sagitta and the nematoid 

 worms and certain Naiada?, Mr. Huxley concludes by observ- 

 ing " that the study of its development can alone decide to 

 which division of the annulose sub-kingdom Sagitta belongs ; 

 but that until such study shall have demonstrated the contrary, 

 he stated his belief that Sagitta bears the same relation to the 

 Tardigrada and Acarida? that Linguatula (as has been shown 

 by Van Beneden) bears to the genus Anchorella, and that the 



