STUDIES ON THE ARACHNID ENTOSTEENITE. 259 



or muscles representing t,he dorsal moieties of the tergo-sternal muscles of tlie 

 somites bearing the first, second, tliird, fourth, and fifth post-oral appendages. 

 IsL, 2s(., 'Ssi., ^st. Ventral processes representing the ventral moieties of the 

 same muscles. Or. Lateral crest developed mainly to support some of the 

 muscles of the appendages. 



PLATE 13. 



Fig. 1. — Dorsal view of enfosternite of the American Limulus (X. poly- 

 phemus), showing the two pairs of long slender autero-lateral processes 

 (2/y. and 3/^.), the stunted muscle-bearing process (1(^.), find the large pos- 

 tero-lateral process {^tg.). (After Lankester.) 



Fig. 2. — Dorsal view of the enfosternite of a Thelyphonid (Mastigo- 

 proctus giganteus) showing the dorsal processes {Itg. to 5/^.), which are 

 considered to represent the dorsal moieties of the tergo-sternal muscles of 

 the second to the sixtii somites of the prosoma ; sa}, supernumerary or addi- 

 tional apopiiysis, which has perhaps arisen from the (ission of the apophysis 

 numbered 2(^. ; a. b., anterior bridge, and p. b., posterior bridge, bounding 

 the large foramen in front and behind; the smaller foramen is shown behind 

 the posterior bridge; l.b., lateral bar, showing perhaps the origin of this 

 portion of the enfosternite from a great longitudinal muscle, or from paired 

 subepidermal ventral entochondrites ; P.F., posterior plate, with frayed edge 

 indicating the attachment of radiating muscles, the suggested homologue of 

 the dorsal portion of the "diaphragm " in the Scorpions (see PI. 14, figs. 21, 

 22); cr., lateral crest. 



Fig. 3. — Dorsal view of the entosternife of a Phrynid (Damon John- 

 stoni), showing tiie duplication of theapophyses numbered lA/.and 3/^.; cr., 

 lateral crest, to which leg-muscles are attached; '\st., anterior ventral 

 apophysis, the suggested representative of the sternal moiety of the tergo- 

 sternal muscle of the second segment of the prosoma. 



Fig. 4. — Dorsal view of entosternife of one of the Mygalomorphse (Ephe- 

 bopus mu rinus), with same lettering as in the last figure, showing the 

 absence of supernumerary apophyses and the presence of a thickened ridge 

 {p. b.), the suggested homologue of the posterior bridge in the enfosternite of 

 Thelyphonus. 



ElG. 5. — Dorsal view of the enfosternite of Stasimopus Sclionlandi, a 

 Mygalomorphous spider of the family Ctenizidse, showing the presence of two 

 supernumerary apophyses {sa."^ and sa.'^) and the fusion of the anterior ventral 

 apophyses {\st.) to form a neural collar. 



I'lG. G. — Dorsal view of entosfernile of Actinopus Wallacei, a Mygalo- 

 morphous spider of the family Actino|)odi(l8e, showing the fusion of the four 

 ventral apophyses to form a subneural arch with perforated walls for the exit 



