ENTOMOSTRACA. 555 



gists. Their limbs are converted into tufts of jointed 

 cirri, and protrude through an opening in the mantle 

 which lines the interior of the shell. The cirri, twelve 

 in number, are covered with cilia, which, when the animal 

 is alive, are in continual motion. The intestinal canal is 

 complete, and the nervous system exhibits the usual 

 series of ganglia, characteristic of the articulate type. 

 The head is marked only by the position of the mouth, 

 and is armed with a pair of jaws, if we may so term 

 the shells. 



BalanidcBy " Sea-acorns," a sessile species, whose curious 

 little habitations may constantly be met with upon the 

 rocks of the sea-shore, and not unfrequently upon many 

 species of marine shells. The shell forms a short tube, 

 and is usually composed of six segments securely united 

 together. The lower part of the tube is firmly fixed to the 

 object on which the Balanus has taken up its abode ; whilst 

 the superior orifice is closed by a movable roof, composed 

 of from two to four valves, between which the little tenant 

 of this curious domicile protrudes his delicate cirri in search 

 of nourishment. In the young state the Balanidce freely 

 swim about, and somewhat resemble the following group, 

 Entomostraca. 



Entomostraca, or Water-fleas, undergo a series of remark- 

 able changes from the moment of their escape from the 

 egg to the attainment of their fully matured form. And 

 it is of the highest interest to remark that, in obedience 

 to a law which, if not universal, is at any rate widely pre- 

 valent in the animal kingdom, these temporary or larval 

 forms are themselves closely analogous to the prefect forms 

 of groups still lower in the scale of existence, so that many 

 of them in their early forms were formerly, before their 

 life-history was known, either classed as distinct species, or 

 placed in a position very far from that which they are now 

 seen to occupy. The embryo of the Shore-crab (Carcinm 

 mcenas) before, and for a short time after, its liberation 

 from the ovum, presents both in size and general outline 

 a strong resemblance to Entomostraca. In this transi- 

 tion stage it was assigned to a distinct genus under the 

 name of Zoea ; and having undergone a still further trans- 

 formation was called Megalopa. In this latter stage it 



