2 Mr. C. Darwin on the Structure and Propagation of Sagitta. 



but I do not suppose that they all belong to the same species : 

 those obtained in lat. 37° to 40° S. appear certainly to be the S. 

 exaptera of D'Orbigny ; and the few following observations, which 

 relate chiefly to then propagation, apply, when not otherwise 

 stated, to this species. M. d'Orbigny and Prof. Forbes have 

 provisionally placed this genus amongst the nucleo-branch mol- 

 lusca ; but the evidence is hardly conclusive. 



Head. — The linear-lanceolate head, which is of a transparent, 

 gelatinous and adhesive textm-e, is separated from the body by a 

 distinct neck. The head when not in action is slightly flattened 

 and of a truncate-conical shape ; when in action its basal part as- 

 sumes a semilunar or horse-shoe form, in the concavity of w^hich 

 lies the longitudinally-folded mouth. On each arm of the fleshy 

 horse-shoe, a comb, formed of eight strong, curved, slightly 

 hooked claws or teeth, is attached. The animal when lively 

 is constantly clasping these bristle-like teeth together, over its 

 mouth ; when clasped together, and the head in a state of inac- 

 tion, they appear to be situated much nearer to the mouth than 

 when their fleshy bases are expanded in action. The middle teeth 

 are the longest ; besides their clasping action and the power of 

 movement in their fleshy bases, each separate tooth can move 

 itself laterally further from or nearer to the adjoining ones. The 

 mouth opens on the oblique surface of a part projecting up, be- 

 tween the two fleshy arms. Close to the mouth there are two 

 other rows of exceedingly minute teeth, which have not been no- 

 ticed by other observers, and which I discovered only with a lens 

 of high power. These two rows of little teeth project inwards 

 and transversely to the two great upright combs of teeth ; so that 

 when these latter are clasped over the mouth the minute teeth 

 cross them, thus effectually preventing any object from escaping 

 which might be caught by the longer curved teeth. I could not 

 see any vestige of eyes or of tentacula. 



Locomotive organs. — The animal moves quickly by starts, 

 bending its body. The two pair of lateral fins and that on the 

 tail lie in the same horizontal plane : viewed with a lens of small 

 power they appear formed of a delicate membrane, but under a 

 lens of ^\jth of an inch focal distance they appear to consist of 

 excessively fine transparent rays, touching each other, like the 

 barbs of a feather, but not, as it appeared to me, actually united 

 by a membrane. The tail, besides being used as a locomotive 

 organ, serves as a means of attachment ; for the animal when 

 placed in a basin of water sometimes adhered by its tail so firmly 

 to the smooth sides, that it could not be detached by a consider- 

 able agitation of the water. Out of the innumerable specimens 

 which I procured, I never saw one fastened by its teeth to the ova 



