AND ENDOSKELETAL SYSTEMS OF LIMULUS. 321 



these are pierced by the genital apertures ; each leads into a duct, which passes upwards 

 and slightly outwards, lying parallel to the " posterior lamellar " muscle, coming nearly 

 up to the thoracic carapace, alongside the sixth coxotergal muscle ; here it breaks up 

 into branches. 



There are a pair of tendinous stigmata, and in all other respects the genital oper- 

 culum is similar to the succeeding appendages. 



IV. Entochondrites. 



1. Prosomatic or Plastron. — This internal skeletal structure (PI. LXXVI. figs. 3, 4, 

 5, 6) is a flat, roughly rectangular, cartilaginous body, with its longer axis directed 

 antero-posteriorly. It lies in the centre of the prosoma, above the mouth and nerve- 

 collar, between the entocoxites, to which a large number of muscles pass from it. 

 Dorsal to it lies first the alimentary canal, and then the anterior aortic trunk. Muscles 

 pass from it to other parts. 



It is convenient for subsequent use in the terminology of the muscles to apply the 

 name " plastron " to the prosomatic entochondrite. 



The general flat surface may be called the " body " of the entochondrite or plastron ; 

 its anterior border is concave anteriorly, and each side is produced forwards as a short 

 stout process, to which various muscles are attached : these may be called the " anterior 

 comua" (Ac.en). 



The front edge is produced laterally into a long slender bar of cartilage, which, rising 

 outwards and upwards, passes between the third and fourth entocoxites ; to the distal 

 end of this process is inserted a short muscle, attaching it to the carapace outside the 

 coxotergal muscles. 



Behind this, and springing close to it, is a second long process ; this passes outwards 

 between the fourth and fifth entocoxites, and like the front one is attached to the 

 carapace by a muscle beyond the coxotergals. These may be called the " lateral 

 comua" (l.c.en). 



The hinder part of the side of the " body " passes outwards, and with the produced 

 posterior edge of the entochondrite forms a " latero-posterior process" on each side 

 (Ip.c.en). 



Posteriorly, in the middle line, is a "posterior process" which rises very slightly 

 above the "body" (p.c.en). 



From the dorsal face of this entochondrite, just behind the base of this hinder lateral 

 cornu, is a short stout " dorsal process " (d.c.en) on each side, which rises backwards, 

 upwards, and slightly outwards. 



To all these processes are attached muscles, some from the thoracic appendages, 

 others going to the carapace &c. 



The microscopic structure of this organ has been described by Prof. Lankester since 



3c2 



