No. 3-] LIMULUS POLYPHEMUS. 469 



VI. The Development of the Branchial Cartilages, 

 Muscles, and Nerves. 



The cartilages in the abdominal appendages of Limulus first 

 appear as solid outgrowths of the ventral wall of the somites. 

 The following descriptions and drawings produced as proof of 

 this statement are based in the main on what takes place in 

 the operculum. The cartilages in the other abdominal append- 

 ages develop somewhat later, but in a very similar manner. 



In an embryo of three abdominal segments, as shown in 

 PI. XXII, Figs. 1-8, where the genital duct is seen as a 

 diverticulum on the median ventral wall of the somite, there 

 is no trace of cartilage in the thick somatic layer of mesoderm. 

 But when five abdominal somites are developed the cartilage 

 can be easily distinguished (PI. XXII, Figs. 9-20, cr). At this 

 time the somite has moved toward the yolk, leaving a large part 

 of the somatic layer behind, as the ring of mesoderm around 

 the base of the appendage. At the same time a transverse 

 plate of cells is formed subtending the ring. This plate of 

 cells is the "Anlage" of the opercular cartilage. Its ventral 

 edge hangs freely into the cavity of the appendage ; its median 

 and lateral ends are continuous with the cells forming the 

 mesodermic ring, while its dorsal edge is continuous with the 

 ventral wall of the somite. The opercular cartilage is therefore 

 derived from the ventral wall of the somite and lies just lateral 

 to the point where the genital duct opens into it. At this 

 time the cartilage cells do not differ in histological appearances 

 from those in the surrounding mesoderm. 



The cartilage of the first gill-bearing appendage develops in 

 a similar manner, but more slowly than that in the operculum, 

 as seen in PL XXII, Figs. 12-16, S0\ There are openings 

 in the anterior wall of the first gill somite that appear to afford 

 a normal communication between the coelom and the surround- 

 ing blood spaces. The posterior wall of the gill contains a mass 

 of ectoderm cells in active division, evidently preparing for the 

 formation of the gill leaves which appear there at a later period. 



The relations of the mesodermic ring, cartilage, genital duct, 

 and somite are shown in cut i. 



