CRUSTACEA. 187 



masked that of some one or other of the higher Crustacea ; and, pro- 

 bably, one of the common species of the Irish coast. To the de- 

 velopment of these, therefore. Dr. Thompson next turned his attention, 

 and he succeeded in hatching the ova of the common crab during the 

 month of June, and found that the young were excluded under the 

 form of the Zoea Taurus^ with the addition of lateral spines to the 

 thorax ; whereupon he concluded that the decapod Crustacea indis- 

 putably underwent a metamorphosis. 



In the year 1829 Dr. Rathke published his Researches on the river 

 craw-fish {Astacus fiuviatilis). In this species the ovum first appears 

 in the shape of a minute transparent vesicle, which afterwards becomes 

 surrounded by a second, forming the membrana vitelli : the nature of 

 the processes effecting this stage appears not to have been observed. 

 The yolk increases in quantity, and is rendered opake by the presence 

 of numerous granules, and changes from the lenticular to the spherical 

 figure ; then the internal minute transparent germinal vesicle quits 

 the centre, and comes into contact with one part of the parietes of 

 the ovum. The colour of the yolk successively changes to yellow, 

 orange, and brown ; the clear vesicle disappears, and the production 

 of the embryo commences. Rathke failed to ascertain what became 

 of the vesicle. The formation of the ovum in the ovary continues 

 half a year. In the month of November the vesicle was visible ; in 

 the ensuing March it had disappeared. 



The ovum escapes into the oviduct by bursting the inner lining of 

 the ovary. It is surrounded by a layer of albuminous matter, and 

 is enclosed within a coriaceous chorion and an irregular deposited 

 nidamental tunic, by which the ovum, after exclusion, becomes at- 

 tached to the ciliated plates beneath the tail of the mother. 



The first appearance of the embryo is as a whitish cloud of inde- 

 terminate form, spreading over the vitellus, and assuming, as it 

 extends, a reticulated appearance. A discoid portion of the layer 

 is defined from the rest, and increases in thickness at its middle part : 

 its longest diameter is about half the radius of the egg. A depression 

 appears in the centre of this, which passes more and more deeply into 

 the vitellus, and the embryonic spot expands at its margins. The 

 patch next grows heart-shaped, and the an- 

 tennae, the labrum, mandibles, and abdomen 

 become simultaneously recognisable. The 

 antennae {fig. 92, b and c), at first short and 

 simple processes, increase in length, and their 

 extremities become notched ; the mandibles 

 (d) also lengthen and enlarge, particularly in 

 their basal portion; tlie labrum (e) recedes 

 from between the anterior antenuce, and takes 



Astacus fluviatilis. 



