NEPHRIDIA AND BODY-CAVITY OF DECAPOD CRUSTACEA. 411 



much enlarged, covering the front part of the ovaries, and 

 running downwards on either side into the cavity which sur- 

 rounds the intestine and liver. The sac extends backwards to 

 near the front end of the pericardial cavity, but it is com- 

 pletely closed, and has no communication with the latter. 



By means of a series of sections through a fully grown 

 adult specimen of Palsem one tes varians, I have ascertained 

 that exactly similar relations exist in that form, and the 

 sections entirely confirm the results of dissection. 



The position of the sac in larvse is also similar to that which 

 it occupies in adults. It is seen in transverse section in figs. 

 6 and 9, c, and in sagittal section in fig. 11, c. It extends 

 from the base of the rostrum to a point in front of the anterior 

 end of the pericardial septum. The sac appears to be com- 

 pletely closed, and in preserved specimens contains a clot, 

 which can generally be distinguished from the surrounding 

 blood-clot, and which contains no corpuscles. 



I have only met with one reference to the existence of this 

 sac. Weldon (No. 20) describes the nephro-peritoneal sac of 

 Palsemon as lying "ventral to the ophthalmic artery and to 

 the median dorsal blood-sinus.-'-' The dorsal sac above described 

 is what is here referred to as the " median dorsal blood-sinus." 

 It does not, however, contain blood. I have been led to this 

 conclusion for the following reasons : — (1) In a large number 

 of series of sections, both of larvse and adults, I have never 

 seen a blood-corpuscle within the sac. (2) The sac is closed, 

 and has no communication with the blood-sinuses of the body. 

 This point can be definitely established for larvee. With 

 regard to adults I cannot make such a positive statement, on 

 account of the difficulty of obtaining a perfect series of un- 

 damaged sections, but I have seen nothing which would sug- 

 gest a contrary conclusion. (3) I have observed carefully 

 and for a long time living larvse under the microscope. If the 

 larvse are lying upon their sides blood-corpuscles are readily 

 seen passing along the cephalic aorta, and corpuscles returning 

 from the head are also observed ; but the latter keep always 

 quite close to the anterior lobes of the liver. The space occu- 



