250 i{. I. POCOCK. 



those from the posterior extremity ruuning backwiirels 

 into the opisthosoma in continuity with the longitudinal 

 muscles of this region. This is well shown in Liuiulus 

 (Benhani, 2) ; in Epeira by Schimkewitsch, and in the 

 specimen of Atypus figured in PI. II, fig. 17. Again^ in the 

 Thelyphonidae, the anterior two thirds of the entosternite, 

 apart from the dorso-lateral apophyses, consist of a pair 

 of stout parallel longitudinal bars, united by an anterior and 

 a posterior transverse bridge, the posterior lobe alone con- 

 sisting of an undivided subcircular plate. 



The solidification of these muscles was no doubt brought 

 about to afford a firm support for the muscles of the proso- 

 raatic appendages. To resist the action of these muscles, 

 which would tend to draw the bars asunder, tendinous bridges 

 were developed across the middle line serving to hold the 

 entire structure in place. As a later development in the 

 Aranese and the Amplypygi, the intervals between the bridges 

 were filled in and the projecting marginal angular crests so 

 characteristic of the entosternite of the Araueie were formed to 

 increase the attachment-area of the leg-muscles. Latent 

 potentiality for transverse fusion between the originally 

 separated right and left halves of the entosternite may be 

 inferred from the fusion of this nature tliat has actually 

 taken place between the ventral apophyses in Ac ti nop us. 



It is possible that the longitudinal muscles have not 

 played so important a part in the formation of the ento- 

 sternite as here suggested. The entosternite may be almost 

 equally well derived on theoretical grounds from dorso- 

 ventral and crural muscles alone, the anterior bars forming 

 the {)haryngeal notch resulting from the fusion of the tendons 

 of the doi'so-ventral muscle ol the second somite with those of 

 the appendages of the second and third somites, and the longi- 

 tudinal direction of the bars being assignable to the forward 

 movement of the second appendage pulling the tendons in a 

 semicircle round the pharynx and brain, the notch thus 

 formed becoming deeper and deeper to accommodate itself 

 to the concomitant backward movement of these organs. 



