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The chief organs of motion are still the two anterior pairs of feet, which are slender and furnished with long setae; the third pair of feet loses its branches, and becomes converted into mandibles destitute of palpi. The labrum acquires a spine directed forward and of considerable size, which occurs in all the Zoeae of allied species.
The heart possesses only one pair of fissures, and has no muscles traversing its interior like trabeculae, whilst in other Zoeae two pairs of fissures and an interior apparatus of trabeculae are always distinctly recognisable.
The second pair of antennae, which, in the various Zoeae always remains a step behind that of the adult animal, is particularly remarkable. In the Crabs the "scale" is entirely wanting; their Zoeae have it indicated in the form of a moveable appendage, which is often exceedingly minute.
Xantho. It is further to be remarked that the Zoeae of the Crabs, as also of the Porcellanae, of the Tatuira and of the Shrimps and Prawns, are enveloped, on escaping from the egg, by a membrane veiling the spinous processes of the carapace, the setae of the feet, and the antennae, and that they cast this in a few hours.
The biramose maxillipedes appear to assist but slightly in locomotion. The forked tail reminds us rather of the forms occurring in the lower Crustacea, especially the Copepoda, than of the spatuliform caudal plate which characterises the Zoeae of Alpheus, Palaemon, Hippolyte, and other Prawns, of the Hermit Crabs, the Tatuira and the Porcellanae.
The addition of fine coloured particles to the water allows this current of water to be easily detected even in small Zoeae. One of these projects upwards from the middle of the back, a second downwards from the forehead, and frequently there is a shorter one on each side near the posterior inferior angles of the carapace.
But except what is common to all Zoeae, and what may easily be explained as being transferred back from a later into this stage, the Zoeae of the Crabs, for example, agree with those of Pagurus and Palaemon in no single peculiarity of structure which leads us to suppose a common inheritance.
The tail, of very variable form, always bears THREE pairs of setae at its hinder margin. The Zoeae of the Crabs usually maintain themselves in the water in such a manner that the dorsal spine stands upwards, the abdomen is bent forwards, the inner branch of the natatory feet is directed forwards, and the outer one outwards and upwards. Tails of the Zoeae of various Crabs. Pinnotheres. Sesarma.
There were, I believe, perfect Insects before larvae and pupae; but, on the contrary, Nauplii and Zoeae far earlier than perfect Prawns. In contradistinction to the INHERITED metamorphosis of the Prawns, we may call that of the Coleoptera, Lepidoptera, etc. an ACQUIRED metamorphosis.* In favour of this view we have:
In the Hermit Crabs a similar, usually moveable, spiniform process occurs as the remains of the scale; their Zoeae have a well-developed but inarticulate scale. A precisely similar scale is possessed by the adult Prawns, in the Zoeae of which it exists still in a jointed form, like the outer branch of the second pair of feet of the Nauplius or Peneus-Zoea.
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