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segments of the 3rd — 7th appendages. Afther the formation of the 

 segment of the 7th appendage, the segment of the 1st appendage is 

 cut off from the 1st segment, and then segments of the 8th, 9th etc. 

 appendages are formed behind the segment of the 7th appendage. 



The 5th appendage is first of all produced, then gradually ante- 

 riorly 4th, 3rd, 2nd, 1st, then gradually posteriorly 6th, 7th etc. All 

 the appendages are postoral in origin. The 7th appendage becomes 

 the metastomum. 



3) Coelomic Cavity. — The coelomic cavity is not produced in the 

 segments of the 2nd, 3rd, and 4th appendages. The cephalic lobe and 

 the segment of the 1st appendage have a common coelomic cavity, 

 which developes along the lateral sides of the stomodaeum, and reaches 

 to the dorsal part through the yolk. In every one of the segments 

 posterior to the segment of the 5th appendage a pair of the coelomic 

 cavities appear, which extend over the yolk and envelope it. The dor- 

 sal ends of all these coelomic cavities as well as the coelomic cavities 

 themselves meet one another in the dorsal median line; but they do 

 not fuse together. Hence a longitudinal median lumen (the dorsal cir- 



culatory vessel) and many lateral slits (the ostia) . The mesoderm belong- 

 ing to the segments of the 2nd, 3rd, and 4th appendages play no part 

 in the formation of the dorsal circulatory vessel. 



When the dorsal circulatory vessel is formed the coelomic cavity 

 of the segments of the 5th, 6th and 7th appendages is divided into 

 2 portions — a ventral, and a dorsal , which now do not communicate 

 with each other. The ventral portion of the coelomic cavity belonging 

 to the segment of the 5th appendage remains as the coxal gland. 



4) Dorsum and Ventrum. — In Limulus there is a distinct line of 

 demarcation between the dorsum and the ventrum. The ventrum is 

 reflexed dorsalwards in the céphalothorax. The céphalothorax of Li- 

 mulus (and of Trilobites also) is composed of 5 lobes, — median (rachis), 

 1st lateral (pleurae), and 2nd lateral (free cheeks). The 2nd lateral lobes 

 are the reflexed portions of the ventrum. In Limulus they are connec- 

 ted, right and left, in front. In the abdomen, the ventrum is not 

 reflexed dorsalwards ; therefore the dorsal surface of the abdomen is 

 formed of the dorsum only and is composed of 3 lobes (the median and 

 the laterals). 



