No. I.] STUDIES ON LIMULUS. 109 



The remaining five pairs of abdominal appendages (Text-fig. 

 6) are alike in form, but decrease in size towards the posterior 

 end of the body. A deep median cleft divides them into right 

 and left halves, between which in the median line is inserted 

 a membranous tongue or median lobe (Text-fig. 6, m.l). The 

 basal portion of each half bears a gill book (Text-fig. ly, g.b.) 

 consisting of numerous overlapping leaflets. The endopodite 

 (?./.) is slender, and produced a short distance beyond the broad 

 exopodite or outer lobe (<?./.). 



2. The Endoskeletal System. 



In Limulus there are a number of cartilaginous bodies which 

 serve for the attachment of muscles. These are the plastron, or 

 endocranium (Text-figs. 2-5 ; Pis. VI and VIII, Figs, i, 3, and 

 4, endo.) ; six small abdominal endochondrites (Text-figs. 5 

 and 6; Pis. VI, VIII, and IX, Figs, i, 3, 4, and 6, a.e.^-'^), one 

 at the base of each pair of abdominal appendages ; and six pairs 

 of branchial bars (Text-figs. 5 and 6 ; Pis. VI and IX, Figs, i 

 and 6, b.c.^-'^) supporting the operculum and gills. Another 

 pair of branchial bars {b.c.'') supporting the chilaria are fused 

 with the endocranium. 



a. The Endocranium. 



The endocranium has been fully described in the first paper 

 of this series. Here we shall merely state that this piece of 

 cartilage serves as a centrum for the attachment of the longitu- 

 dinal abdominal muscles (Text-figs. 5 and 6 ; Pis. VI, VIII, and 

 IX, Figs. I, 4, and 6, I. a.m.), tergo-plastrals (Pis. VIII and IX, 

 Figs, 4 and 5, d.l.p.t.), tergo-proplastrals (Text-fig. 8 ; Pis. VI, 

 VIII, and IX, Figs, i, 4, and 5, t.p.m.''-^), veno-pericardiac 

 muscles (Text-fig. 4 ; PI. VIII, Fig. 4, v.p.m.^^), and numerous 

 muscles inserted upon the coxopodites of the appendages, from 

 second to the seventh, inclusive. 



A pair of bars (Text-figs. 4 and 5 ; Pis. VI, VII, and VIII, 

 Figs. 1-4, b.c.7) of capsuliginous cartilage, identical with that of 

 the branchial bars of the abdominal appendages, are fused. 



