NEPHRIDIA AND BODY-CAVITY OF DECAPOD CRUSTACEA. 413 



sides, appears to be a more primitive form of the same 

 process. 



In embryos of Palserao net es, in which the cephalic aorta 

 is already formed, the latter is not surrounded by a single 

 layer of cells, but the nuclei are in reality arranged in two 

 layers, an internal and an external. This may be seen from 

 fig. 13, A and b, which are drawn from a late embryo of 

 Palaemonetes; but the arrangement in two layers becomes 

 even more obvious when a series of sections is examined. 

 Fig. 13, B, represents the condition of things along the greater 

 part of the length of the aorta, but a few anterior sections 

 appear as in fig. 13, a, where a tongue of mesoderm extends 

 downwards between the liver lobes. This condition of things 

 will be explained at a later stage. In both figures (fig. 13, a 

 and b) the nuclei m! belong to the internal layer, the nuclei 

 ri' to the external. 



Before the time of hatching arrives, the cells which surround 

 the aorta become considerably enlarged, and give rise to the 

 appearance of solid masses of mesoderm upon either side. 

 Fig. 14, E, may be taken to represent the condition of things 

 throughout the greater part of the length of the aorta just 

 before the larva leaves the egg. In this figure the blood is 

 coloured black, the central mass {^Ao. c.) being in the cavity of 

 the aorta, the remainder being in the cavity of the body which 

 lies between the liver lobes (/i.) and the ectoderm {ect) ; mes. 

 represents the mass of mesoderm on either side of the aorta. 



Soon after hatching a change takes place, which leads to the 

 formation of the dorsal sac. Fig. 14 represents sections through 

 different regions of the thorax of a larva very soon after it has 

 become free. The second section drawn (fig. 14, b) passes 

 through the region of the paragnaths. The cells of the external 

 layer (cf. fig. 13, b) are here seen to have drawn away from 

 those of the internal layer which form the walls of the aorta, 

 leaving a clear space (c), which extends almost completely 

 around the latter. A few sections further back this cavity 

 begins to close, until the condition represented in fig. 14, c, is 

 reached. A comparison of this section with those which pre- 



