NEPHRIDIA AND BODY-CAVITY OF DECAPOD CRUSTACEA. 421 



at about the level of the shell-gland — that is to say, in the 

 region of the second maxillse (ib., pi. iv, fig. 1). In 

 Neb alia Claus figures a section which passes through the 

 valves which separate the heart from the aorta, and this also 

 passes through the point where the nerves of the second 

 maxillae are given oflp from the nerve-cord (No. 8, pi. xi, fig. 11). 

 Claus, however, states that this section is somewhat oblique, 

 sloping downwards and backwards, so that we must place the 

 anterior end of the heart at least as far forwards as the front 

 end of the segment of Maxillae II. 



In a Stomatopod larva (No. 2, pi. iv, fig. 8) the front end of 

 the heart is figured as being slightly anterior to the shell- 

 gland. 



In young larvse of Palaemonetes, as I have already shown, 

 the dorsal sac ends in the posterior part of the region of the 

 second maxillae ; but in all these cases it must be borne in 

 mind that, owing to the moving forward of the appendages 

 towards the mouth, the actual segment to which any part of 

 the cephalic aorta or heart belongs can only be very approxi- 

 mately determined. Considering, however, the facts adduced, 

 we may, I think, fairly say that in no Crustacean does the 

 heart extend in front beyond the region of the second maxillae, 

 and that there is therefore no evidence to show that heart 

 and pericardium have ever been differentiated in segments 

 anterior to this. 



Summary of Conclusions. 



1. The green gland of Palaemonetes (and Palaemon) at 

 the time of hatching of the larva has not developed a lumen, 

 although the external opening can be detected. When the 

 larva leaves the egg the lumen commences to open, and the 

 gland consists of an end-sac and a U-shaped tube, of which the 

 distal portion gives rise to the bladder. The bladder then 

 enlarges greatly, growing at first inwards towards the middle 

 ventral line, then upwards, within the oesophageal nerve-ring 

 and anterior to the oesophagus, to the middle dorsal line, 



