Fig. 70 (repeated). Daphnella.
A, summer ovum, with
an oil-globule (Oe). B, winter
ovum.
In almost all the Daphnids the winter egg develops into a perfect animal just like that to which the summer egg gives rise, although it no longer receives any nourishment after it passes into the brood-chamber. But it receives a larger supply of yolk on this account, so that the nutritive provision within the egg is sufficient to develop the perfect animal. There is only one exception to this, and it is of special theoretical interest, because it shows more plainly than any other fact that the greater or less degree of condensation in the ontogeny depends upon the combined effect of the external conditions of life. The largest of the Daphnidæ, Leptodora hyalina, a beautifully transparent inhabitant of our lakes, which measures about a centimetre in length (Fig. 110), also emerges from the summer egg as a perfect animal, but from the winter egg, which floats freely in the water and has only a small provision of yolk, it emerges as a nauplius, which then undergoes larval metamorphosis before it becomes a perfect animal (Fig. 111).
Fig. 110. The largest of the Daphnids (Leptodora hyalina), with summer ova (Ei) beneath the shell (Sch). I-IX, the appendages. II, the oars (second antennæ) which always remain biramose in Daphnids. sb, setæ. ov, ovaries. Schl, œsophagus. Ma, stomach. a, anus. H, heart. Au, eye. nG, natural size.
Fritz Müller concluded from the repetition of the nauplius form in all orders of Crustaceans that the primitive form of the Crustacean must have been a nauplius, and that from it all the modern Crustaceans must have evolved phyletically by the addition of segments varying in number and differentiation. Now, however, it is doubted whether there ever were nauplioid types capable of reproduction. But even if the nauplii only represent what have been the larval forms from very early times, they are equally important in illustrating the relations between ontogeny and phylogeny; they at any rate represent the primitive pre-cambrian larval form from which all modern Crustaceans are derived. This is borne out not only by the facts to which we have already referred, but also by those Crustacean-groups which have diverged far from the usual Crustacean habit and type.
Fig. 111. Nauplius larva from the
winter egg of Leptodora hyalina; after
Sars.
Thus the sessile Cirrhipedes, with their mollusc-like shells, their soft, unsegmented bodies, degenerate heads, and their twelve vibratile food-wafting limbs, emerge from the egg as nauplius larvæ. But the remarkable parasites on the shore-crabs and the hermit-crab deviate much further from the type of the rest of the Crustaceans, for they hang like a sac or formless sausage-like soft mass to the abdomen of their host, growing into it by fine, pale, root-like threads, through which they suck up the blood of their hosts (Fig. 112, C. Sacc.). They possess neither head, nor thorax, nor abdomen, not even an indication of segmentation, no limbs of any kind, neither antennæ, nor mouth parts, nor swimming-legs. Nevertheless they are Crustaceans; indeed, we can say with certainty that they belong to the order of Cirrhipedes, for they leave the egg in the form of a nauplius larva (A), with 'horns' on their carapace which no other forms except themselves and the Cirrhipedes possess. That they are of the same stock as these is also proved by their further development, for the nauplius grows first, just as in the case of the Cirrhipedes proper, into a 'Cypris-like larva' (B), so called because it bears a certain resemblance to the Ostracods of the genus Cypris, and only from this point do their paths of development diverge. The Cypris-like larva of the true Cirrhipedes settles down somewhere, attached by its antennæ; it grows, and its body becomes that of the perfect Cirrhipede; but the Cypris-like larva of the Sacculinæ bores its way into the inside of a crab or hermit-crab, at the same time losing its limbs, segmentation, and its chitinous covering; and within the body of its host it is transformed into the sac-like organism we have already described. After a time it emerges again on the surface, and remains attached to the abdomen of its host (Fig. 112, C. Sacc.), drawing its nourishment from the blood which it sucks up by means of its numerous delicate roots (W, W).
