STUDIES ON THK AUACHNID KNTOSTERNTTE. 249 



once suggest their currespoudeiice with ihe siinihirly placed 

 scars upon the sterna of the opisthosoma in, e.g., Phryuus, 

 which admittedly indicate the ventral attachments of the 

 tergo-sterual muscles. Dissection, however, shows that the 

 scars on the sternum are the points of insertion, not of 

 muscles, but of the tendinous processes which project down- 

 wards from the lower surface of the eutosternite. These 

 tendinous processes are, I believe, the solidified 

 ventral moieties of the primitive p rosomatic tergo- 

 sternal muscles. Apart from other considerations, their 

 muscular origin is attested by their representation in the 

 Arachnomorpha) (e.g. Ctenus) by muscles passing from 

 the lower surface of the eutosternite and affixed by a fibrous 

 strand to corresponding points on the sternum. 



The dorsal moieties of these same muscles are represented, 

 I think, by the paii-ed tendinous apophyses springing from 

 the upper side of the eutosternite and passing into muscular 

 fibres which fasten them to the lower side of the carapace. 

 There is never a sternal scar near the base of the sixth append- 

 age in the Mygalomorpha), and no apophyses, either dorsal 

 or ventral, of any appreciable length, corresponding to 

 this limb, on the entosteruite. Short chitiuous ridges are 

 observable, however, on this plate in the appropriate 

 positions. These considerations suggest the suppression of 

 the fifth pair of apophyses as a concomitant, no doubt, of the 

 constriction as the last somite of the prosoma. 



The acceptance of this view of the nature of the dorsal and 

 ventral processes of the eutosternite carries the conclusion 

 that a large part of this plate in the Aranea) results from the 

 tendinous solidification of five pairs of tergo-sternal muscles. 



Evidence that a share in the formation of the plate has 

 been contributed by at least one pair of longitudinal muscles 

 is supplied by the following facts. Apart from the appen- 

 dicular muscles, which originally took a transverse direction, 

 both the anterior and posterior extremities of the euto- 

 sternite afford support to muscles; those from the anterior 

 bars passing forwards to the front wall of the prosoma, 



