Today, July 19th only warranted three words in Darwin’s diary:
“A calm day.” (July 19)
I guess when seasickness is your bane, a few words say a lot – namely “relief”.
Darwin did take the time to do a little “biologizing”, presumably collecting samples with his plankton net. His Zoological Notebook goes into some detail about one of the critters he caught – the “jellyfish” Liriope tetraphylla.
In the field, Darwin referred to this animal by the names Diancea exigua or Pelagia (both names seem to be out of use today). However, upon more careful examination back in England, this animal was classified as a Liriope tetraphylla, – which appears to be the only existing member of the Geryoniidae family.
Liriope are from the Phylum Cnidaria which includes corals, jellies, anemones and hydra (see The Immortal Hydrazoa). Cnidaria have two basic body forms – the free-swimming medusa (with a mouth opening on the bottom of the animal), and the stationary polyp – the animal that spends most of its live tethered to the ground and has a mouth on the “upper side”. Some of the Cnidaria have a life cycle that incorporates both phases. For example, hydrozoa spend a large part of their life as floating medusa which eventually reproduce sexually producing juveniles who attach to a surface as a polyp.
This particular “jellyfish”, belongs to the class Hydrozoa (which includes everything from the very small hydra, to the enormous Portuguese Man-o-war). Liriope is on the smaller end of that spectrum. I believe (though again he leaves off the units) that Darwin suggests the liriope were 0.2 inches in diameter.
Sketches of liriope from Darwin’s notebook (from the Complete Works of Charles Darwin Online). Darwin was not much of an artist and sketches in his notebooks seems to be far less common than one would expect.
Here are a few observations Darwin made (the total would fill the better part of a page) on this day in 1832. Some refer back to his sketch. I like the abbreviated and annotated nature of his field notes – I can relate to that (as can a lot of field scientists, I’m sure). It gives them that authentic feel.
“Back convex, octagonal.— at each angle a projecting fibril, which is highly flexible & contractile, & capable of seizing any object (?) — These are of two sorts (Fig 3)4, one shorter thicker & striated transversely; the other long transparent within about seven little balls.— [note (a)] Are these minute balls Ova? & the shorter fibrils ovaria without the eggs.— these shorter are exactly equal either in order (Vide Figure) or in size.— [note ends]”
“These fibrils are seated on a tube running round the edge.— which also is contractile.— In centre is cylindrical hollow projecting tube, terminated by an organ capable of assuming various shapes.”
“In Fig: (2) on the convex surface there may be seen a faint cross of fibres: it would appear to be the muscular organ of contraction.”
” From the octagonal margin (& not drawn in plate) there depends a delicate membrane which is slightly contractile at its inferior margin, forming a sort of bag.— In this shape I found the animal, but being kept it altered shape of body very remarkably & I think this latter the most natural.— The dorsal surface became much inflated, but was protruded through the octagonal margin on the other & inferior surface.— & the depending veil was turned upwards.— so that the central tube was now in the inside of body (In short the animal turned itself outside inward, every part except the tube.) ”
“Medusæ moving by sudden contractions.— Body highly transparent colourless.”
In other words this (from the Encyclopedia of Life):
I believe the 4 large organs seen inside the body are the gonads.
Surprisingly there is not a lot written about this particular species online (many just taxonomic information). One variety (a subspecies?) that lives in deeper water gets all the press because it is bioluminescent. One example from Japan, is shown below (from Wikipedia Commons):
(Note the long “beaded” tentacles and the central tube – both of which Darwin describes).
Darwin makes no mentions of the fact that these little jellies were glowing, but then again, it was probably the daytime. (RJV)