Entomostraca (Water-Fleas). 449 



the young undergo from the time of their escape from the ovum 

 to the attainment of their fully matured specific 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 prevalent in the 

 animal kingdom, these temporary or larval forms are themselves 

 closely analogous to the perfect forms of groups still lower in 

 the scale of nature ; so that many of these undeveloped beings 

 were formerly, before their life-history was known, classed as 

 distinct species, in a position very far from that which they are 

 now seen to occupy. These changes have been closely studied 

 in the common shore crab [Car emus moenas). The embryo of 

 this species before, and for a short time after, its liberation 

 from the ovum, presents both in size and general outline a 

 strong resemblance to some Entomostraca. In this stage it was 

 of old assigned to a distinct genus under the name of Zoea, and 

 having undergone a still further transformation was called 

 Meg atop a. In this latter stage it puts on somewhat the appear- 

 ance of the <c lobster- crab" (Qalathea), and after another step 

 attains its true crab form, being the highest development of 

 which it is capable. These changes are not produced gradually, 

 but by a succession of " moults," the animal becoming for a 

 time sluggish, casting its hard covering, and reappearing in a 

 new guise. It is a well-known fact that the after-growth of 

 Crustacea is carried on by the system of moulting — the hard 

 calcareous case of the animal preventing its growth in any other 

 fashion. And as in the higher orders of Crustacea, so also 

 amongst the Entomostraca, are transformations of this kind 

 constantly apparent. For example, let us take the same species 

 which we have just noticed as a characteristic instance of repro- 

 ductive energy — Cyclops quadricomis. When first born, these 

 little creatures (figs. 1, 2) are totally unlike their parent, being 

 of an ovoid shape, and having only two short antennas and two 

 pairs of feet; but by three moultings the animal reaches its 

 perfect form, having then two pairs of antennas and five pairs 

 of feet, besides which the body is seen to be divided into seve- 

 ral distinct rings or segments. These changes will be best 

 understood by reference to the plate (figs. 1, 2, 3, 4), the 

 drawings in this instance being copied from Dr. Baird's work. 

 The length of time occupied by these changes varies much 

 according to the temperature. In February and March, Jurine 

 found it to extend over twenty-eight days, whereas Dr. Baird 

 has seen the process completed in eleven days during the warm 

 weather of June. 



Among the many interesting phenomena connected with 

 the reproductive process in the Entomostraca, there remain 

 one or two which we cannot pass over without brief notice. 

 It is in members of this group that several of the most un- 



