AND ENDOSKELETAL SYSTEMS OF LIMULUS. 323 



VI. Tendinous Stigmata. 



These are invaginations of the cuticle near the base of the abdominal appendages, 

 one on each side 6f the middle line on the posterior face of each appendage : the 

 hollow invagination is as much as one inch in depth (PI. LXXXIV. figs. 4, 5, st). 



The stigma is at first composed of two or three layers of epidermal chitin ; then, as 

 we pass inwards, we find it invested by fibrous connective tissue forming a tendon, to 

 which the branchio-thoracic muscle is attached. 



Muscles. 

 I. The Longitudinal Muscles. 



The Prosoma and the Mesosoma. — These cannot be separated, as the principal muscles 

 rise in the prosoma, but mainly lie in the mesosoma. 



No. 1. The Dorsal Entapophysio-plastral. — This rises on the dorsal face of the 

 posterior process of the plastron or prosomatic entochondrite, beneath 54, and passes 

 over the base or attachment of the dorsal plastro-tergal muscle (55), directly back- 

 wards into the mesosoma, below the intestine, just on each side of the median plane 

 (PI. LXXIII. figs. 1, 2, and PL LXXVI. fig. 4). On reaching the mesosoma it gives 

 off a bundle (83) to the third entapophysis, to which structure it is attached on the 

 anterior edge. The main muscle then passes on, giving off a bundle successively, to 

 each of the following entapophyscs (84, 85, 86), nos. 4, 5, 6, the main bundle (87) 

 running on to the last entapophysis — the metasomatic. In its course it includes the 

 vertical mesosomatic muscles (12) between the main bundle and each branch to the 

 entapophyses. 



No. 2. The Ventral Entapophysio-plastral. — This rises from the dorsal face of the 

 plastron, nearly covering this structure, passing beneath no. 54 ; like the preceding, 

 just below which it passes, it runs into the mesosoma, breaking up into bundles, which 

 go to the entapophyses 3, 4, 5, and 6. It likewise ends in the metasomatic (seventh) 

 entapophysis. Just outside its branches rise the veno-pericardiac muscles (68). 



The branch to the third entapophysis is lettered 103 in the Plates. 



The branches to the fourth, fifth, and sixth entapophyses are lettered 104, 105, 106 

 respectively. 



The terminal slip (107) is inserted into seventh entapophysis. 



No. 3. The Ventral Longitudinal. — This is a much smaller muscle than either of the 

 preceding, lying on the abdominal floor. It is shown in PI. LXXV. fig. 3. It rises 

 from the dorsal face of the plastron underneath the origin of no. 2. On reaching the 

 mesosoma it gives off a bundle (no. 69) to the second mesosomatic entochondrite. It 



