No. 3-] LIMULUS POLYPHEMUS. 465 



tubes ; towards their lateral ends their somatic walls are much 

 thickened to form the muscular tissues of the appendages. 



The chelarial segment, it will be observed, contains a well- 

 defined abdominal somite, although it is smaller than those in 

 front or behind it. This fact shows conclusively that the che- 

 laria are true appendages, having the same morphological value 

 as the other appendages of the body. 



The opercular somite is the largest, the following ones 

 gradually diminish in size from before backwards. 



Figs. 8 and 9, PI. XXII, represent longitudinal sections 

 through an older specimen. They show four fully formed 

 abdominal somites, with a fifth one just separating from the 

 anterior end of the primitive streak. 



The somites grow over the surface of the yolk in a postero- 

 lateral direction, till they unite in the median dorsal line. The 

 circular bands of mesoderm thus formed become smaller and 

 smaller in diameter from the opercular segment towards the 

 posterior end of the body. 



At an early period the distal ends of all the somites become 

 continuous with a thickened band of proliferating cells that 

 forms a well-defined margin extending round the whole embry- 

 onic area (see Patten, '90, p. 373). 



The somites of the opercula and gill-bearing segments persist 

 as the well-defined blood sinuses leading from the abdominal 

 appendages to the pericardium. 



V. The Development of the Genital Ducts. 



The genital ducts arise as diverticula from the median ven- 

 tral side of the opercular somite, and extend towards the median 

 line along the base of the opercular cartilages. They lose Q.) 

 their connection with the somites as soon as two or three gill 

 leaves are formed on the first branchial appendage, and then 

 remain in a very rudimentary condition until after the second 

 larval stage. The distal ends of the diverticula finally unite 

 with shallow invaginations of the ectoderm, while their proxi- 

 mal ends unite in a manner not yet determined with the genital 

 organs. 



