United States or Vanuatu ? Vote for the TOP Country of the Week !
This appears to be only inconsiderable in the common Lobster, the young of which, according to Van Beneden, are distinguished from the adult animal, by having their feet furnished, like those of Mysis, with a swimming branch projecting freely outwards. From a figure given by Couch the appendages of the abdomen and tail also appear to be wanting.
We may see, without further discussion, how the representation given by Claus of the development of the Copepoda may pass almost word for word as the primitive history of those animals; we may find in the Nauplius-skin of the larvae of Achtheres and in the egg-like larva of Cryptophialus, precisely similar traces of a transition towards direct development, as were presented by the Nauplius-envelope of the embryos of Mysis and the maggot-like larva of Ligia, etc.
We might urge that, according to this proposition, provisional organs as the first produced must exceed the later-formed permanent organs in importance. But let us stick to the Crustacea. In Polyphemus Leydig finds the first traces of the intestinal tube even during segmentation. In Mysis a provisional tail is first formed, and in Ligia a maggot-like larva-skin.
Do not Nauplius-like states in this case also precede the Zoea? The last segment and the last two pairs of feet of the middle-body are wanting. The developmental history of Mysis, the near relationship of which with the Shrimps and Prawns has recently again been generally recognised, has been described in detail by Van Beneden. So far as I have tested them I can only confirm his statements.
How earlier young states may gradually be completely lost, is shown by Mysis and the Isopoda. Like the spinous processes of the Zoeae, the chelae on the penultimate pair of feet of the young Brachyscelus are to be regarded as acquired by the larva itself.
Does it not necessitate that we should unite as the descendants of the same primitive ancestors, Mysis with the Isopoda on the one hand, and on the other the rest of the Podophthalma with the Amphipoda? I think not. Such a necessity exists only for those who estimate a peculiarity at a higher value because it makes its appearance at an earlier period of the egg-life.
As we have already seen, Mysis and the Isopoda depart from all other Crustacea very remarkably by the fact that their embryos are curved upwards, instead of, as elsewhere, downwards. Does not so isolated a phenomenon as this, it might be asked, in the sense of Darwin's theory, indicate a common inheritance?
The primitive body includes the segments which the naupliiform larva brings with it out of the egg; it is afterwards divided, by the younger sections which become developed in its middle, into the head and tail. Like the antennae, the caudal feet may also become the bearers of high sensorial apparatus, as is shown by the ear of Mysis.
These are the antennae and mandibles. The egg-membrane now bursts, before any internal organ, or even any tissue, except the cells of the cutaneous layer, is formed. The young animal might be called a Nauplius; but essentially there is nothing but a rough copy of a Nauplius-skin, almost like a new egg-membrane, within which the Mysis is developed.
The five new pairs of feet are biramose, the inner branch short and simple, the outer one longer, annulated at the end, furnished with long setae, and kept, as in Mysis, in constant whirling motion. The heart acquires new fissures, and interior muscular trabeculae.
Word Of The Day