191 



NATURAL HISTORY. 



" venous sinuses." Here the blood is collected, and thence passes up into the gills to be oxygenated, 

 after which it is returned to the heart. 



With the exception of the CIERIPEDIA (see p. 196), the two sexes in the Crustacea are always 

 distinct. Among many of the Crustacea, the antennae and the great claws are specially developed in 

 the male. This is the case in the claws of Corystes and Macrophthalmus and many others, whilst in 

 Cyclops one of the antennae in the male is specially modified for clasping the female. In the Crab 

 the female lays eggs which have been already fecundated. In the Lobster the eggs are fecundated 

 after their extrusion from the ovaries, whilst adhering to the abdomen of the female. 



In nearly all the Crustacea the young undergo a series of metamorphoses after they quit the egg. 

 This is especially the case with the truly marine forms. Among the DECAPODA (Crabs and Lobsters) 

 some few species certainly quit the egg in the same form as their parents, with apparently the same 

 number of jointed appendages to their bodies. This is the case with the River Cray-fish with 

 sevei*al Land Crabs (Gecarcinus, &c.), with a species of Dromia, and with the common Garden 

 Wood Louse (Oniscus, Porcellio, and Armadillo), which likewise nearly resemble their parents at birth. 

 One of the most interesting series of metamorphoses undergone by any of the Crustacea is 

 that passed through by the young of the common Shore Crab (Fig. 8). In this species the 



metamorphosis is a perfectly 

 gradual one, and dissimilar as 

 is the zoea when it quits the 

 egg from the adult animal, yet 

 nevertheless the change at each 

 moult is so small that it is only 

 by a comparison between the 

 earliest and the las'; stages that 

 the amount of the change which 

 has actually taken place can 

 be fully appreciated. Thus, in the zooid state 

 the young Crab has fixed eyes without eye- 

 stalks, a long body, destitute of any appen- 

 dages ; it has no walking legs, but it is a free- 

 swimming form, performing its locomotion 

 with its maxillipedes, or jaw-feet (m), which are 

 greatly developed, serving as a pair of long 

 oars, the long hairs of which probably fulfilling 

 the office of branchiae. Even when by succes- 

 sive moults the true ten-footed character is 

 seen, the young nevertheless present at first a 

 greater likeness to the long-tailed Lobsters 

 than to the short-tailed Crabs. These tran- 

 sient characters displayed by the larvae are 

 found to be persistent in many of the lower and simpler forms now living, and they also charac- 

 terised some of the ancient fossil Crustacea found in the Silurian formation. Thus it may be 

 seen that the stages of development of the individuals of to-day are but a reflection of the life- 

 history of the class in past, geological time. 



Still more strange are the changes undergone by the brood of some Prawns of the genus Penwus 

 observed by Fritz Miiller (Fig. 9- 1 , 2, 3, 4, 5). These quit the egg with an unsegmented ovate body (2), 

 a single eye in front, and three pairs of swimming feet, of which the first pair are simple, and the other 

 two pairs branched. In this stage, called the ttauplius, there is no trace of a carapace, the paired eyes 

 are wanting, and also the masticating organs, the mouth being covered by a helmet-like hood. After 

 several moults the nauplius becomes a zoea (3), being furnished with maxillae and two pairs of jaw- 

 feet. The third stage of the same Prawn (4) exhibits still more remarkable changes, the paired eyes, 

 the segments of the thorax, the rudiments of the feet are seen, all the appendages of the mouth and 

 head can be counted, and the plates of the tail sprout forth. And now another great change takes 



8. THREE STAGES IN THE METAMORPHOSIS OF THE 



COMMON SHORE CRAB. (Carcinus mcenas.) 



1, a newly hatched zoea ; 2, a more advanced Bta^c "f same, 3, side view of 

 still older larva (Jfegalopa stage). (After C. tipence Bate.) 



