Mr. C. Darwin on the Structurer and Propagation of Sagitta. 3 



of pelagic animals^ or to other bodies, as M. d^Orbigny has ob- 

 served in some of his species. 



Internal viscera. — Within the body, in the same plane with the 

 longitudinally folded mouth, there is a flattened tube or cavity, 

 which in the specimens obtained in lat. 18° S. I observed had the 

 power of contracting and enlarging itself in different parts, and 

 within it there was a distinct peristaltic movement. Within this 

 cavity in the S. exaptera I could clearly discern in the posterior 

 half of the body a delicate vessel, which I presume is the intes- 

 tine, for it appeared to terminate on one side of the body at the 

 base of the tail. I could discover no vestige of a nucleus, of 

 branchiae, of a liver, or of a heart. In some exceedingly young 

 specimens, however, just liberated from the egg, there was a di- 

 stinct pulsating organ (as will hereafter be mentioned) in the an- 

 terior part of the body. 



Propagation. — The state of the reproductive system varies much 

 in animals caught at the same time. Taking a specimen with 

 this system in a high state of development, the tail, or the taper- 

 ing part of the body into which the intestinal tube does not pe- 

 netrate, is seen to be longitudinally divided by an exceedingly 

 delicate partition, and to be filled with a pulpy finely-granular 

 matter. The column of matter on each side of the central divi- 

 sion also appears (but whether really so I do not know) to be di- 

 vided, making altogether four columns, as is shown in the diagram. 

 The whole of this matter is in a state of steady and regular cir- 

 culation, something like that of the fluid in the stems of the 

 Char a. The matter flowed upwards in the two outer columns, 

 and downwards towards the point of the tail in the two middle 

 columns. The circulation in the up-flowing columns was most 

 vigorous on their outer sides ; and in the down-flowing columns 

 on their insides, that is, on each side of the central partition : this 

 would be accounted for, if we might suppose that the two sur- 

 faces of the central partition were covered with cilia, vibrating in 

 a direction opposite to that in which other cilia situated on the 

 inside of the membrane forming the tail were also vibrating. The 

 stationary condition of the granular matter between the two 

 streams, travelling in opposite directions, perhaps gives the ap- 

 pearance of the partition on each side of the central one. The 

 circulation at the base of the tail was twice as rapid as it was 

 near the apex : where most rapid I found that a granule travelled 

 over the 2 jo*^ ^^ ^^ uioh. on the micrometer in five seconds ; 

 allowing for the slower rate in other parts, I calculated that in 

 an individual, the tail of which was /^ths of an inch in length, a 

 granule performed its entire circuit in about six minutes. I could 

 distinctly follow the granules descending one column, turning the 

 angle, and again ascending. In specimens with the reproductive 



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