I am currently rereading Darwin’s The Descent of Man: and Selection in Relation to Sex and found an interesting statement.
In a subsection, called Lower Stages in the Genealogy of Man, to chapter VI Darwin says:
Many years ago Prof. Goodsir perceived that the lancelet presented some affinities with the Ascidians, which are invertebrate, hermaphrodite, marine creatures permanently attached to a support. They hardly appear like animals, and consist of a simple, tough, leathery sack, with two small projecting orifices. They belong to the Molluscoida of Huxley–a lower division of the great kingdom of the Mollusca; but they have recently been placed by some naturalists amongst the Vermes or worms. Their larvæ somewhat resemble tadpoles in shape,21 and have the power of swimming freely about. Some observations lately made by M. Kowalevsky,22 since confirmed by Prof. Kuppfer, will form a discovery of extraordinary interest, if still further extended, as I hear from M. Kowalevsky in Naples he has now effected. The discovery is that the larvæ of Ascidians are related to the Vertebrata, in their manner of development, in the relative position of the nervous system, and in possessing a structure closely like the chorda dorsalis of vertebrate animals. It thus appears, if we may rely on embryology, which has always proved the safest guide in classification, that we have at last gained a clue to the source whence the Vertebrata have been derived. We should thus be justified in believing that at an extremely remote period a group of animals existed, resembling in many respects the larvæ of our present Ascidians, which diverged into two great branches–the one retrograding in development and producing the present class of Ascidians, the other rising to the crown and summit of the animal kingdom by giving birth to the Vertebrata.
What makes this interesting is footnote 21 which says:
I had the satisfaction of seeing, at the Falkland Islands, in April, 1833, and therefore some years before any other naturalist, the locomotive larvæ of a compound Ascidian, closely allied to, but apparently generically distinct from, Synoicum. The tail was about five times as long as the oblong head, and terminated in a very fine filament. It was plainly divided, as sketched by me under a simple microscope [bold mine - afarensis], by transverse opaque partitions, which I presume represent the great cells figured by Kowalevsky. At an early stage of development the tail was closely coiled round the head of the larva.
Note that Darwin says he sketched the critter. I have been searching The Complete Works of Charles Darwin Online and have been unable to find the sketch. So I am hoping the collective wisdom and abilities of my readers may succeed where I have failed…
Update 1: Phillip IV has found it for us:
It comes from Charles Darwin’s zoology notes & specimen lists from H.M.S. Beagle and is described by Darwin as follows:
V. Pl: 7
¼ & 1/3 focal distance
April 2d.– Aggregate body, oblate spheroid, seated on a footstalk, which tapers at root to a fine point: gelatino-membranous, external parts yellowish transparent, internal reddish orange. Formed from the aggregate of numerous animals, the bodies of which point towards common centre or footstalk, hence the central ones are longest & others gradually decrease in length towards the sides.– They adhere side by side, & from each a narrow elastic ribbon goes proceeds to the footstalk & passes down to the root.– External surface slightly mamillated, with apertures each of which is common to the branchial cavity & other orifices of |168| each animal.– Orifice bean shaped, edge slightly fringed.– near to convex side, there is a white internal mark formed of collection of dots.– [note (b)] I do not know what to make of these white dots, which are universally present: they can easily be separated.– numbers also occur about the region of the stomach, but in this latter place they are not constant in numbers or site.– [note ends] this side is external to pole of sphere on inferior.– From these white marks & shape, consistence & colour, body resembles some fruit, such as a Strawberry: size of large specimen, breadth of sphere .8: height .6; length, including head & stalk, about 2 inches.– Grows on the leaves of the Fucus giganticus.– .separated
Body of animal may be is divided into two parts;– branchial cavity,– & abdominal viscera.– Branchial cavity bell-shaped, furnished with slightly tubular lip, on which are two rows of differently sized papillæ, about 16 in number (?): these project across the expanded aperture.– [note (c)] The papillæ resemble on a small scale those on the arm of an Asterias.– When the animal is undisturbed, the branchial cavity is widely open & a slight circulation of water may be perceived at the aperture.– [note ends] the sides on mantle is [are] divided into two halves [note (a)] not separated or cut [note ends] by vessels running up on each side; in both there is a most beautiful & symetrical trellis work of branchiæ. They It consists of 5 concentric rows (or combs) of parallel filaments, which are vertical; they are attached at each extremity to mantle; in middle rows they are attached to bands.– Perhaps they might be described as four concentric bands with filaments above & below, but where opposite united.– The filaments towards each end of the comb decrease in size length.– When the animal is undisturbed the two upper & larger rows can only be seen, the others were discovered by difficult dissection; On these filaments, with a high power, a rapid vibrating motion is visible, as if of ciliæ, clearly a function of respiration.– |169|
[next two pages have been crossed through diagonally in pencil. CD P. 169 commences]
The vessels which divide the mantle & the two sets of trellis work; are very clear near the aperture but by no effort could I trace them to a junction with others of the viscera.– On the external side, a clear space runs up, to which the concentric bands unite.– & in this is a vessel, containing another, which runs up seems to unite to the white space by branchial aperture.– [note (a)] Is it impossible that this vessel is connected with base of tentacula or papillæ & from thence leads to mouth of œsophagus at base of branchial cavity.– animal would then live solely by absorption!? — it is the simplest method of joining the vessels: [note ends] I could not see any orifice.– I could trace these vessels down the side of cavity, but not across it, which direction it must pursue if it unites to any of the viscera.– On the anterior & superior side there is a minute vessel, which seems also to terminate in a yellow dot by branchial aperture & right opposite to white space.– the interval between this vessel & intestine is so small, that I have no doubt that it is the anus.–
Near base of Branchial cavity the œsophagus enters, & proceeding descending a short distance, bends nearly at right angles & passes under & through the liver.– forming together large dark reddish orange unequally sided oval.– the intestine taking a sweep ascends close by the œsophagus to near aperture of bran Mantle.– between the stomach & bend of intestine the heart lies, appears elongated & very transparent; pulsating strongly; I could trace the oscillations to within the Branchiæ, I imagine therefore the circulation is simple:– Resting on & beneath the intestine & stomach: there is a large sack of white pulpy matter, which generally often is divided internally into a star like mass.– it is in this state when most undeveloped. |170| When a little more advanced, the white matter is collected into globular ova.– from the centre of this sack a vessel descends & bending suddenly ascends close by the intestine & therefore on the outside of animal.– I could trace it as far as the end of intestine, but from these vessels & œsophagus all lying close between the trellis work of branchiæ, I could by no effort trace them to their orifices.–
This last vessel is clearly the oviduct: I will first describe the most extraordinary ovules & then the process of generation.– From the first rudimentary globular collection of white matter, they pass into (2nd state) defined reddish orange spheres: 3rd with a point on one sides: 4th.– surrounded by clos a transparent band in which are transverse opake partitions: 5th a rounded oblong, with central dark mass enveloped by gelatinous transparent matter, furnished with a long tapering tail.– Tail has numerous transverse partitions, & in 4th state was curled around ovum.– it terminates by a fin[e] hair & in different times is either filled with homogeneous matter or opake partitions.– Total length .11; breadth of head .015, so that the tail is about 5 times as long as head.– it Ovule is capable of rapid vibrating motion & hence progressive: it is evidently a young Synoicum in search of a Fucus on which the tail will be fixed & become a footstalk: From appearance of head it is a single |171| animal.– This gemmule resembled in its habits some Infusoria3, as Circaria.–
In the described ovarium, only those ova in 1st & 2d state are found.– For independent of this organ, there are, when the aggregate body abounds with ovules, two intestine shaped sacks, longer than the body & attached near to extremity of intestine, or supposed anus.– I never saw these except when with eggs. At lower extremity the ovule appears to be much in same state as in the true ovarium, but at the upper end or mouth they are in state 4th: & some even with when fused their tails uncurl: I should suppose that ovules pass down the oviduct & enter the two additional ovaria & there remain till ready to become independent animals.– In same proportion as the two additional ovaria contain many ova, the central one contains few & the whole animal becomes exceedingly shrunk; so that the aggregate body is of a darker reddish orange & appears to be composed of intestine shaped sacks with ova.– The number of eggs in each animal vary according to its size, so that those near the footstalk only contain a few, whilst the large central ones very many.– The ovules in same aggregate body were nearly in same state.– some with central ovarium only containing white pulpy matters, others filled with large bright coloured ovules: |172| Aggregate bodies of different sizes (therefore ages?) contained ovules; otherwise I should have thought from shrunk state of bodies that after parturition animals had died.– [pen changes] The footstalk is enveloped in strong membrane & consists of the elastic ribbons & some granular balls, the nature of which I am ignorant of, enveloped in gelatinous matter:
I have called this animal Synoicum, as in external characters being nearest, but it is evidently distinct.– In the anatomy the generation is very curious & one more instance of ovules having a motion of which the parent animal is not possessed.– the number of tentacula round edge of mantle, & the curious trellis work of Branchiæ are all remarkable facts.–
Plate 7, Fig 5. represent, but stiffly drawn, an animal with branchial cavity expanded: tentacula about aperture: the vessels within branchiæ are disjoined from fault of observ: heart lies on under surface just by function of stomach & intestine, not (drawn): ova just formed: Fig 6 is the vessels which lead towards collection of white dots, with upper band of branchiæ of the two trellis works.– Fig 7. one whole set of trellis work expanded; miserably drawn, filaments longer & far more numerous & regular.– Fig: 8.– Ovule in 4th state: Fig: 9:– Ovule in 5th state.– Fig 10, piece of tail much magnified:– |173|
2 Ascidiacea, a tunicate or sea squirt. CD has well observed, independently of its discovery by Milne Edwards a few years earlier, the brooding larvae typical of the animals in cold waters, and the way in which the tails of the tadpoles curve around the trunk. But it is their heads rather than their tails that become attached to their supports.
Note: The footnote is by Richard Keynes, the editor of the volume.