NEPHRIDIA AND BODY-OAVITY OF DECAPOD CRUSTACEA. 415 



represented in fig. 14, c. The condition with two lateral cavi- 

 ties (fig. 14, d) then appears, and persists in eleven sections 

 in the region of the first maxillse. The appearance seen in 

 fig. 14, B, in which no cavity is yet formed, runs through the 

 next nine sections in the region of the second maxillge to 

 the point where the bands of connective tissue suspend the 

 intestine. 



In a larva only a few hours older than the one just described, 

 what appears at first sight to be a very considerable develop- 

 ment has taken place. From the region of the mouth to that 

 of the second maxillse the parts under consideration have the 

 appearance represented in fig. 16, a, a few sections only at the 

 posterior end presenting that shown in fig. 16, b. There is 

 now one cavity, which surrounds three sides of the aorta and 

 extends along its whole length. But no great rapidity of 

 growth has been necessary to produce this marked result. By 

 comparing the sections of fig. 14 with those of fig. 16 it will be 

 seen that what has taken place is that the protoplasm of the 

 cells, instead of being gathered into masses around the nuclei, 

 has spread out into a thin sheet, thereby enlarging con- 

 siderably the cavity, and drawing its walls away from the wall 

 of the aorta. 



The sections at the posterior end of the sac in this and the 

 following stages are of special interest, because in them the 

 cavity is divided by a vertical septum (fig. 16, b, s.) into two 

 lateral portions. This fact tends to confirm what is already 

 apparent from fig. 14, d, of the previous stage, that the cavity 

 of the dorsal sac is formed by the union of two lateral cavities, 

 which lie on either side of the cephalic aorta. 



The further development of the dorsal sac consists mainly 

 of an increase in its size. Later stages are seen in fig. 9 and 

 fig. 6, c. Its walls are very thin, and the nuclei spindle- 

 shaped. At its posterior end the sac grows backwards as a 

 pair of lobes, which are not attached to the wall of the aorta, 

 and which extend as far as the front end of the pericardium. 

 A section through these lobes is seen in fig. 17. This poste- 

 rior portion of the dorsal sac becomes very much enlarged in 



