EMBRYOLOGY OF THE PODOPHTHALMATA. 113 



setting up a current of water through them. An ovum, when 

 newly deposited, is found to consist of a colourless transparent 

 envelope full of transparent fluid of a tint varying, as we have 

 seen, in different species. This envelope, or membrane, is 

 continued into a strong viscid ligature, which is apparently twisted ; 

 and as these ligatures unite they become stronger and thicker, 

 ultimately forming the stout peduncle which attaches them to the 

 basal joint of the swimmeret, and which supports the whole 

 group. 



The first indication of the development of the egg is the 

 granular appearance that the yelk assumes, and its separation from 

 the envelope ; gradually the outline of the enclosed Zooea becomes 

 defined, and the yelk is then enclosed in the large cephalo-thorax. 



At this stage the most prominent feature is the eye, which 

 gives the ova a most remarkable speckled appearance, even when 

 seen without the aid of the microscope. 



In the mature egg the abdomen of the Zooea is closely folded 

 on the sternum of the cephalo-thorax, and the limbs lie in close 

 contact with the antennse, antennules, and mouth organs. When 

 the Zooea leaves the egg the envelope of the latter is simply a 

 collapsed and crumpled membrane, and in this respect resembles 

 the ova of many of the Lepidoptera. 



The larval, or Zooea, forms of the stalk-eyed Crustacea are 

 most remarkable in structure, and until a comparatively recent date 

 were regarded as a distinct order of animals, or rather as aUied to 

 the Entomostraca. When first hatched their eyes are sessile, 

 their cephalo-thorax large, more or less round in form, and, in 

 many genera, armed with large curved spines. The abdominal 

 segments are long and simple, terminating in a remarkable filamen- 

 tous tail ; the Zooea of Lithodes maia is particularly curious in this 

 respect, its tail development presenting a broad, fan-shaped 

 expanse of branching filaments of most delicate and beautiful 

 structure. The swimming feet are absent, but the ambulatory 

 feet are developed into limbs armed with setae, thus supplying the 

 necessary natatory organs ; as the true swimmerets appear, these 

 others gradually assume the structure of walking-appendages. 

 These larval forms, in successive moults, assume the eyes fixed 



