NEURAL ARCHES AND BRANCHIAL CARTILAGES. 307 



serve for the attachment of segmental muscles. At no time in its phyllogenetic 

 history has it any functional connection, or relation to the endoderm, or to the ali- 

 mentary canal. 



II. THE NEURAL ARCHES. (Figs. 70, 75, 78, 209.) 



The neural arches (endochondrites of Lankester) are six small plates of 

 fibre-cartilage. They lie on the neural surface of the cord beneath the integu- 

 ment, to which they are attached near the base of each pair of gills. Each arch 

 is concave on its inner surface and partly surrounds the nerve cord, holding it 

 firmly in place. (Figs. 70, 75, 78.) The opercular neural plate is typical; it is 

 rectangular, and its flat neural surface is indented by two pits, from which arise 

 a pair of muscles attached to the inside of the operculum. A pair of anterior 

 and posterior processes serve for the attachment of strands from the longitudinal 

 muscles of the abdomen. On the sides of the arch is a pair of processes which 

 project haemally and a little outward and backward, one on each side of the 

 ventral cord. They furnish attachment for a pair of hsemo-neural muscles that 

 are inserted on the hsemal side of the carapace, just median to the entopophyses. 



The neural arches of Limulus may be regarded as the precursors of the neural 

 arches of vertebrates, with which they agree in location, and in their general form 

 and function. No other invertebrate is known to have neural plates of this 

 character. 



III. BRANCHLA.L CARTILAGES. (Figs. 78, 2og, 210, 211.) 



There are seven pairs of branchial cartilages in Limulus. The most anterior 

 pair are two small bars arising from the inner surface of the chilaria, and attached 

 to the posterior margin of the endocranium. (Fig. 215.) The remaining six 

 pairs arise from the base of the abdominal appendages, and go to the corresponding 

 entopophyses. (Fig. 209.) 



The branchial cartilages arise at an early embryonic period as clearly de- 

 fined outgrowths of the walls of the mesoblastic somites. (Fig. 210.) Their 

 union with the epideimis is secondary, and they are in nowise derived from 

 chitenous ingrowths of the epidermis. 



The branchial bars serve for the attachment of the flexor and extensor 

 muscles of the gills, and for a small muscle, the internal branchial, arising from 

 the corresponding neural plate. The opercular bar is the largest. * In a small 

 male, it is about 25 mm. long, oval in cross-section, and about 6 mm. by 3 mm. 

 in diameter. The remaining bars decrease gradually in size to the posterior end 

 of the series. 



A band of cartilage, similar histologically to that of the branchial cartilages, 

 extends from the distal end of one entopophysis to the next, uniting the haemal 

 ends of the gill bars. (Fig. 209.) 



