Before I delve into Darwin’s observations on the carnivorous arrow worm (as promised yesterday) let me include a couple of lines from today’s relatively routine diary entry:
“We have made an excellent run of 70 miles to day.— Indeed the breeze to my taste was much too good, as it prevented us from attempting to land at Cape Corrientes, which we doubled at Noon.— We sailed very close to the shore, & it was very interesting viewing the different countries as we rapidly passed on.— North of Corrientes, a dead level line of cliff takes the place of the sand hillocks.— The cliff is perpendicular & about 30 feet high, & with a few exceptions is continued all the way South of the Cape.— From the mast-head a great extent of flat Pampas was seen without any break or elevation.— To every ones astonishment there was near the promontory of Corrientes an Estancia.— Cattle were very abundant near the house, & the place looked prosperous.— (Note in margin: We have heard they have 50000 head.) Two or three men on horseback were watching us with great interest: so we hoisted our pennant & colours, & doubtless for the first time they had ever been seen in this sea.— This farm must be about 200 miles from any town, & the greater part of the interval consists in desert salt plains.— There cannot easily be imagined a more desolate habitation for civilized man.” (Aug 25)
After the voyage was over Darwin had a lot of data to work with and wrote a series of papers and articles about his geological and biological discoveries. One paper, written in 1844, was about the arrow work (Genus Sagitta) and refers back to the observations he made in the summer and fall of 1832 . The paper is only 6 pages long and makes for a quick read. if you are interested in reading one of Darwin’s early scientific papers, here is the reference:
Darwin, C., 1844, Observations on the Structure and Propagation of the Genus Sagitta, Annals and Magazine of Natural History, 13:1-6 (click here to a link to the article in pdf format (including sketches))
Sagitta maxima (from NOAA)
So what is an arrow worm? Arrow worms belong to their own phylum – Chaetognatha. That says a lot, because it means that they are a relatively unique form of life. To put this in perspective, all vertebrates are in the same phylum (the Chordata) which includes birds, fish, dinosaurs, amphibians, mammals, etc. The Sagitta that Darwin describes is a genus within the phylum Chaetognatha.
Like many animal phyla, the Chaetognatha go back to at least the Cambrian era (roughly half a billion years ago). Fossil arrow worm ancestors are found in some of the famous fossil beds of the Cambrian, including the Burgess Shale in Canada and the Maotianshan shales in China.
Arrow worms are almost all transparent. This might explain Darwin’s interest, as he could easily observe their inner workings and sketch their internal organs. These little critters (ranging from about 2-120 mm) are also quite abundant in surface waters, so he must have collected a lot of them in his plankton net. They are hermaphrodites, which is why the ones Darwin observed all contained ova.
In the microscopic world of the surface waters of the ocean, Chaetognatha are the “lions”. They hunt plankton with their compound eyes and eat them with their “toothed” mouths. If you were a 5 mm long critter living in the oceans, you would not want to see an arrow worm! As a human, you don’t have to worry much (unless there are predatory sharks around).
Species of arrow worm – Spadella cephaloptera (from Wikipedia Commons)
While on the voyage Darwin wrote about 4 pages of observations on arrow worms in his Zoological Notebook between August 24 and Sept 4, 1832. These notes seem to be the primary source of his paper 12 years later. For the rest of the post I’ll share a few of Darwin’s observations. I’ll quote his 1844 paper, as the Zoological notes are a little harder to follow (they are field notes after all).
On the occurrence of Sagitta:
“The species of this genus are remarkable from the simplicity of their structure, the obscurity of their affinities, and from abounding in infinite numbers over the intra-tropical and temperate seas. … Scarcely any pelagic animal is more abundant: I found it in lat. 21° N. in the Atlantic, and again off the coast of Brazil in 18° S.; between latitudes 37° and 40° S., the sea, especially during the night, swarmed with them.”
On newly discovered teeth:
“Close to the mouth there are two other rows of exceedingly minute teeth, which have not been noticed 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.”
On locomotion:
“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 … 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 considerable agitation of the water.”
On the “viscera”:
“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 intestine, 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 branchiæ, of a liver, or of a heart.”
On reproduction:
“When the tail is filled with vigorously circulating matter two large cul-de-sacs or gut-shaped ovaries are invariably present, extending, as represented (o o) in the diagram, from the base of the tail along each side of the intestinal tube. These are filled with ova, which in the same animal are in different stages of development, and vary in length from 1/100th to 1/50th of an inch; their shape is pointed oval (Plate I. fig. B [below]), and they are attached by the pointed end in rows to the sides of the ovaries: those of full size are detached by a very slight touch. When the ovaries contain many eggs nearly perfect (but not at other times), a small conical and apparently perforated protuberance can be seen on each side (A A) of the body, through which without doubt the eggs are expelled.”
Darwin sketch of an arrow worm (Sagitta species) from his 1844 paper referenced above:
Darwin wraps up by encouraging further study of this interesting form of life noting, “I will conclude by expressing a hope that these few observations on the propagation of this curious genus may aid more competent judges than myself in ascertaining its true affinities”.
Finally – because there is a web page on everything… if you want to see info on all species of arrow worms visit the Chaetognatha of the World site. (RJV)
The Beagle Project 
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