EXPLANATION OF PLATES XXVIII AND XXlX—contiimed. 



Fig. 5. — Entosternite of bird's-nest spider (Mygale, sp.), dorsal face. 



Fig. 5 a. — The same, neural face. 



Fig. 6. — Entosternite of a Scorpion {Buthis, sp.), posterior face. 



Fig. 6 a. — The same, neural face. 



Fig. 7. — Entosternite of Linmlus poli/phemus, neural face. Compare 

 with Fig. 5 a and Fig. 6 a. 



Fig. 8. — A single lamella of the right lamelligerous appendage of the 

 eleventh segment of Butlius Kochii, formed by two closely adherent 

 plates : a. b., the base or line of attachment. Note the marginal setae. 



Fig. 9. — A single lamella of the right lamelligerous appendage of the 

 eleventh segment of Limulus jiolyphemus, for comparison with Fig. 8 : a. b., 

 the base or line of attachment. Marginal setae are present, but more 

 numerous than in Fig. 8. 



Fig. 10. — View of the posterior face of the conjoined pair of lamelliger- 

 ous appendages of the eleventh segment of Linmlus polyphemus : st., soft 

 sternal lobe or plate which unites the two appendages ; md., median process 

 or soft papilla of the sternal plate ; epst., epistigmatic sclerite ; stg., para- 

 branchial stigma (invagination to give atta-'^hmeut to the thoraco-branchial 

 muscle) ; app. m., protractor muscle of the appendage, seen through the 

 the soft integument ; I., branchial lamellae ; I.', the proximal lamella ; 

 app?, the proximal division of the appendage carrying the lamellse and 

 extending beyond them ; app}, app?, app.*, second, third, and fourth 

 sclerites, forming tiie jointed axis of the distal prolongation of the appen- 

 dage ; ex., exite or outer ramus of the appendage. 



Fig. 11. — View of the pair of lamelligerous appendages of the ninth 

 segment of Limulus polyphemus, seen from in front. The chitinised integu- 

 ment has been removed from the surface of the proximal portion of the 

 appendages, so as to expose the bases of the hollow lamellae, and the soft 

 integument of the median sternal area; and the retractor muscles have also 

 been removed so as to expose the inner face of the corresponding integu- 

 ment of the posterior face, and the insertions of the thoraco-branchial 

 muscles : st., sternal lobe ; stm., thoraco-branchial muscle of the left side ; 

 Bl., bases of the laminae, open to the branchial blood-vessels. 



Fig. 12. — A semi-diagrammatic view of one of the respiratory appendages 

 of a Scorpion, to show BL, the bases of the lamellae exposed, as in BL, fig. 

 11, by the removal of the integument of the axis, the remnants of which 

 are seen at m. The drawing further shows the gradual narrowing of the 

 bases of the laminae in the series as the distal region is approached until 

 the free projecting ])ortiou of the axis (w) is reached ; /., proximal lamella. 



Fig. 13. — A tendon-sac of Limulus polyphemus detached from the para- 

 brauchial stigma, the homologue of the pulmonary sac of Scorpio. 



PLATE XXIX. 



The drawings on this plate are diagrammatic, and illustrate the hypo- 

 thesis as to the derivation of the lamelligerous appendages of Limulus and 

 Scorpio from a common ancestral form. All the figures, except 4', 5, 6, 

 present the appendages as seen when the ventral surface of the animal is 

 facing the observer. 



Fig. 1. — Hypothetical intermediate form. R. axis of appendage; L 



