1892.] Cavity of the Larva of Palaemonetes varians. 341 



and second maxillae, 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 egg, 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 m axillae, whilst in the region 

 of the second maxillae no cavity had yet opened, and solid masses of 

 cells still lay upon either side of the aorta. 



The farther development of the dorsal sac consists mainly of an 

 increase in its size. At its posterior end it grows backwards 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 peri- 

 cardial 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 sections confirm 

 the accounts given by Milne Edwards* and Glaus. t There are two 

 main sinuses, which run along its length : a dorsal sinus, in which 

 the intestine lies, and a ventral one, which contains the nerve cord. 

 These two sinuses are generally separated by masses of muscle, but 

 they communicate at intervals by means of lateral sinuses. 



Theoretical Considerations. 



The state of the body cavity in the anterior region of the thorax 

 of Palcemonetes may be compared with that of Peripatus, as described 

 by Sedgwick,J at the time when the dorsal portions of the meso- 

 blastic somites have attained their maximum development. Bearing 

 in mind that the dorsal sac of Palcemonetes has been formed by the 



* Milne Edwards, ' Histoire naturelle des CrustaceV Paris, 1834. 



t Glaus, C., " Zur Kenntniss der Kreislaufsorgane der Schizopoden und 

 Decapoden." ' Arb. Zool. Inst. Wien,' vol. 5, 1884. 



J Sedgwick, A., " The Development of the Cape Species of Pertpatus." Parts 

 I IV. ' Quart. Journ. Micr. Sci.,' vols. 2528, 1885-88. 



YOL. LII. 2 A 



