Body- Cavity of the Larva o/Palasmonetes varians. 239 



rounding the latter are arranged in two layers, an internal 

 and an external. Before the time of hatching arrives the cells 

 of the external layer enlarge considerably and give rise to the 

 appearance of a solid mass of cells upon either side of the 

 aorta. The dorsal sac is formed by the hollowing out of 

 these masses of cells. Two lateral cavities are thus formed, 

 which are separated by the aorta. The protoplasm of the 

 cells lining these cavities, which is at first gathered into 

 masses around the nuclei, then spreads out into a thin sheet, 

 drawing away from the lower portion of the aorta, and causing 

 the two lateral cavities to unite ventrally and so form a single 

 sac. In the region of the first and second maxillaj all the 

 stages of the process just described may be seen. In the 

 region anterior to this I have never actually observed the 

 stage with two lateral cavities, the two having always united 

 ventrally ; but I do not doubt that the process is here the same 

 as in the posterior region. 



In one series of sections of a larva, preserved very soon 

 after it had left the &gg, the cavity was formed upon either 

 side of and below the aorta, as far back as the anterior end of 

 the segment of the first maxillae, where for one section it was 

 almost completely closed. Behind this the condition with 

 two lateral cavities was found and persisted through the region 

 of the first maxilla?, whilst in the region of the second maxillas 

 no cavity had yet opened, and solid masses of cells still lay 

 upon either side of the aorta. 



The further development of the dorsal sac consists mainly 

 of an increase in its size. At its posterior end it grows back- 

 wards in a pair of lobes, which extend as far as the front end 

 of the pericardium. 



The Posterior Region of the Thorax. — The central and 

 lateral cavities are here similar to those of the anterior region, 

 whilst dorsal to them the pericardial chamber lies. This 

 chamber is separated from the central body-cavity, as is 

 already well known, by the pericardial septum, and it contains 

 the heart. The genital organs are situated at the front end 

 of the pericardium, immediately below the pericardial septum. 

 In the just-hatched larva these consist of two masses of cells 

 with large nuclei, each mass being enclosed in a sheath of 

 mesoderm. I have not detected any trace of the genital ducts 

 at this stage. 



The Abdomen. — With regard to the abdomen, my sec- 

 tions confirm the accounts given by Milne-Edwards * and 



* Milne-Edwards, ' Histoire Naturelle des Crustac^s/ Paris, 1834. 



