Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/678

Rh also found an Australian Dromia protecting its brood on its false abdominal feet, the young differing from the parent only in point of size. FIG. 25. Freshwater Cray-fish from the Mississippi River. (Morse.) The young of the terrestrial Isopoda(Oniscus,Porcellio, and Armadillo) likewise nearly resemble their parents at birth. In the king-crabs (Limulus) the young undergo their principal moults before quitting the egg, when they differ bat little in aspect from the adult, The metamorphosis undergone by the common lobster appears to be but slight. The young, according to Van Beneden, are distinguished from the adult by having their feet provided like those of Mi/sis with a swimming branch projecting freely outwards, whilst the abdominal and caudal appendages are undevel oped. In nearly all the marine Crustacea the young quit the egg in the condition of zoea. We are acquainted with many examplas iu all three divisions of the Decapoda. Bat we are indebted to Mr C. Spence Bate for the most complete series of observations on the development of one species, the common shore-crab, (Cctrcinus m&amp;lt;xnas) from the zoea to the sexually mature animal. 1 He has shown that in this species the metamorphosis is a perfectly gradual one, and that, dissimilar as is the zoea when it quits the egg from the adult animal, yet neverthe less the change at each, moult is so small that it is only by a comparison between the earliest and the last stages that we perceive tha amount of the change which has actually -taken place. &quot;The most important peculiarities,&quot; writes Fritz Miiller, &quot;which distinguish this zoea- brood from the adult animal are as follows. The middle body (thorax), with its appendages, tho.ie five pairs of feet to which these animals owe their name of Decapoda, is either entirely wanting, or scarcely indicated ; the abdomen and tail are destitute of appendages, and the latter consists of a single piece (fig. 26). The mandibles, as in the Insecta, have no palpi. The maxillipeds, of which the third piir is often wanting, are not yet brought into the service of the mouth, but appear in the form of biramose natatory feet. Branchiae are wanting, or where their first Fir.. 2fl. Zoea of Common Shnre- rudirnerits may bs detected as small crab, OwfttiM I*&amp;lt;M, Penn. sp, In verrucifonn prominences, these are its first stage &amp;lt; Spence 1!ate &amp;gt; dense cell-masses through which the blood does not yet flow, and which have therefore nothing to do with respiration. An inter change of the gases of the water and the blood may (and no doubt does) 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. &quot;They consist, exactly as described by Leydig in the Daphnice, 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. A con stant current of water passes beneath the carapace from behind forwards, maintained as in the adult animal by a foliaceous appen dage from the second pair of maxillfe. 1 On the development of Decapod Crustacea, Phil. Trans., 1858, pi xl.-xlvi. p. 589. &quot; The zoeee of the crabs are usually distinguished by long, spini- form processes of the carapace (fig. 27). One of these projects up- Fro. 27. Zoea of Common Shore-Crab in its second stage. r, rostral spine ; s, dorsal spine ; m, maxillipeds ; /, buds of thoracic feet; a, abdomen. (Spunce Bate.) wards from the middle of the back, a second downwards from the forehead, and frequently there is a shorter one on each side near tho Fig. 29. Fig. 28. Fig. 30. FIG. 28. Zoea of Porcellana stellicolia, F. Mull., mag. 15 diam. FIG. 29. Zoea of Hippa eremita, mag. 45 diam. FIG. 30. Zoea of Herm it-Crab, magn. 40 diam. (Fritz Miiller.) posterior inferior angles of the carapace. But in the zoea of Maia,