Branchiae are wanting,
or where their first rudiments may be detected as small verruciform
prominences, these are dense cell-masses, through which the blood does
not yet flow, and which therefore have nothing to do with respiration.
An interchange of the gases of the water and blood may occur all over
the thin-skinned surface of the body; but the lateral parts of the
carapace may unhesitatingly be indicated as the chief seat of
respiration. They consist, exactly as described by Leydig in the
Daphniae, of an outer and inner lamina, the space between which is
traversed by numerous transverse partitions dilated at their ends; the
spaces between these partitions are penetrated by a more abundant flow
of blood than occurs anywhere else in the body of the Zoea. To this may
be added that a constant current of fresh water passes beneath the
carapace in a direction from behind forwards, maintained as in the adult
animal, by a foliaceous or linguiform appendage of the second pair of
maxillae (Figure 18). The addition of fine coloured particles to the
water allows this current of water to be easily detected even in small
Zoeae.
(FIGURE 17. Zoea of a Marsh Crab (Cyclograpsus ?), magnified 45 diam.
FIGURE 18. Maxilla of the second pair in the same species, magnified 180
diam.)
The Zoeae of the Crabs (Figure 17) are usually distinguished by long,
spiniform processes of the carapace.
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