Last time, I wrote about Julian Huxley’s 1912 book, The Individual in the Animal Kingdom, and his use of the volvocine algae as an example. I liked most of what he had to say, though I took issue with his assertion that
…all the other members of the family except Volvox…are colonies and nothing more—their members have united together because of certain benefits resulting from mere aggregation, but are not in any way interdependent, so that the wholes are scarcely more than the sum of their parts.
This is, of course, a matter of how we define a multicellular organism, but I think any definition that excludes, for example, Eudorina, is not a very useful one.
This time, I’ll look at the rest of what Huxley had to say about the volvocine algae, most of which is about Volvox:
Though, as we have said, Volvox is obviously related to Gonium and the others, it is separated from them by somewhat of a gap.
In the first place, it contains, instead of sixteen or even sixty-four cells, a vast number, mounting up in some species to twenty thousand (see frontispiece).
As with the Gonium figure, Huxley doesn’t list A. Lang in the Literature Cited. The only relevant pre-1912 publication I can find is Arnold Lang’s 1888 book, Lehrbuch der vergleichenden Anatomie, which includes this figure:
Sexual, hermaphroditic colony, according to Cienkovsky and Bütschli, and somewhat schematized. S male gametes (spermatozoa), O female gametes (eggs).
The Lang figure shows a hermaphroditic sexual colony, but I can easily believe it’s the model for Huxley’s asexual Volvox globator.
All these cells are inter-connected by fine strands of protoplasm passing through their party-walls and they are arranged in a single layer on the outside of a sphere whose inner parts are filled with a very fluid jelly, so that the Volvox-colony has what we may call an internal medium of its own. Finally, and this is where Volvox has made the great advance, the cells are not all alike. Most are of the type already seen in Gonium and characteristic of the family; these row the colony through the water, steer it, and feed it. Amongst them, in the hinder half of the sphere, are larger cells, lacking flagella and eye-spot, and connected by very numerous strands with their neighbors,
‘Their oarsman-brothers, by whose toil, safe fed
And guarded safe, they lived a charmèd life
Within their latticed crystal, peaceably.’
What the…did he just drop a poem into the middle of a discussion of cellular differentiation? No credit is given, so I have to assume that this is a J. S. Huxley original.
And what do they do in return? Now is discovered the skeleton in the flagellated cells’ cupboard—they cannot reproduce the colony. They are sterile, and must leave reproduction to the big lazy-seeming cells who are only lazy, however, because they must store up food-materials to start the new colony fairly on its way. The grow and grow, bulge inwards, and finally come to float free in the centre space, where they still grow, meanwhile dividing up into a number of cells. In the end, they become perfect miniature colonies, burst out of their parent and swim happily away.
Volvox is thus a real individual; of the two kinds of cells each has given up something the better to fulfil its own special duty. There is division of labour, and, from the point of view of the species, each kind is meaningless without the other.
The division of labour in Volvox is that usually first seen in compound individuals—between the reproductive functions on the one side and all the rest on the other. In other words, one sort of cell is concerned entirely with the species, the other entirely with the separate individuals of which the species consists; to use the current phraseology, the one sort is germinal, the other somatic. The word somatic opens up another view: Volvox is the first organism which, in the ordinary sense of the word, has a mortal body. Its substance is not passed on unimpaired from individual to individual, but with each act of generation the major part must die, sacrificed for the greater efficiency of the race.
A Chlamydomonas cell has no dead ancestors. Each of its ancestors grew to maturity and divided into daughter cells, effectively becoming its daughters. The same could be said of Gonium: each cell divides to produce a daughter colony, and each colony results from an unbroken lineage of cells that survived to reproductive maturity. A colony might die due to misfortune, but successful reproduction leaves nothing behind: the colony doesn’t produce daughters and die, it becomes its daughters. Only with the advent of soma did death become inevitable. In fact, we might think of Volvox as analogous to a semelparous plant: its reproduction is the beginning of the end.
In Volvox, this body consists of but one sort of cell: in all the organisms usually known as Metazoa there are at least two sorts, if not more. Besides the division of labour between germ and soma, there is developed another in the soma itself, at the first between protective an nutritive cells, the one forming an outer covering round the other, which in its turn surrounds an internal cavity. But even if Volvox only possesses species-individuality, the individuality is none the less real; and the fact that in the family Volvocidae we can positively affirm that the step from an aggregate to a higher individual has actually taken place, is one of the most important in biology.
Huxley, J. S. 1912. The Individual in the Animal Kingdom. (Cambridge University Press).
Lang, A. 1888. Lehrbuch der vergleichenden Anatomie. (Verlag von Gustav Fischer).