AND ENDOSKELETAL SYSTEMS OF SCORPIO. 343 



attached ventrally to the posterior margin of the coxal sclerite of the sixth appendage, 

 and dorsally to the arthrodial membrane between the carapace and the first tergite of 

 the mesosoma ; it forms a septum between the prosoma and the mesosoma (see wood- 

 cut, fig. 1, and explanation). From the plastron there are several processes which are 

 similar on each side. The anterior processes (ap) are long and run forwards ; each at 

 its anterior end (a) is attached by fibrous tissue to the ventral surface of the posterior 

 end of the corresponding "cornu" of the preoral entosclerite. A lateral median 

 process (Imp) starts from the side of the plastron and rims out laterally on each side. 

 A posterior process (pp) runs from the posterior end of the plastron, being fused for 

 part of its way with the fibrous flap. There is also a small pair of anterior processes 

 on the subneural portion of the plastron, the " anterior subneural processes " (asp). 



Suprapectinal Chondrite or Second or Mesosomatic Entochondrite. — There is a second 

 and much smaller entochondrite in the segment of the pectines, which is a simple 

 fibrous band lying ventral of the nerve-cord (as do the mesosomatic entochondrites of 

 Limulus), to which several muscles are attached. 



Muscles. 

 The muscles of the Scorpion may be classified as follows : — 

 I. Longitudinal Muscles. 



a. Prosoma. 



b. Mesosoma. 



c. Metasoma. 



II. Dorso- Ventral Muscles. 



a. Prosoma. 



b. Mesosoma. 



c. Metasoma. 



III. Muscles attached to the Plastron. 



IV. Muscles attached to the Suprapectinal Entochondrite. 

 V. Muscles attached to the Preoral Entosclerite. 



VI. Muscles attached to the Appendages. 

 VII. Muscles attached to the Epimeron. 

 VIII. Muscles attached to the Pharynx. 

 IX. Muscles attached to the Pericardium. 



a. Mesosoma. 



b. Metasoma. 



I. Longitudinal Muscles. 

 All the segments of the mesosoma and metasoma are moved on one another by longi- 

 tudinal muscles. In the prosoma, however, as there is but one tergite, so there is but 

 vol. xi. — part x. No. 5. — May, 1885. 3f 



