2 48 \i. I. POCOCK. 



weldino- tog'etlier of the external skeletal elements to form a 

 compact ine-xpaiisible wliole^ the function of these muscles as 

 dilators and contractors of the prosoma would cease, leaving 

 them available for other purposes if reqiiiied. 



The fusion of tergites to form a carapace, acconij)anied no 

 doubt by the partial or complete cessation of function of the 

 longitudinal and vertical muscles, took place, as may be seen, in 

 the Trilobites, before the five pairs of post-oral appendages of 

 the prosoma were set apart as the exclusive organs of locomo- 

 tion and prehension. This specialisation, demanding an in- 

 crease in the size and strength of the limbs in question, would 

 be advantageously accompanied by an increase in the area for 

 the attachment of the enlarged and subdivided muscles that 

 control them. This area might be supplied, in the first in- 

 stance, by the fibrous solidification of the central portion of 

 the tergo-sternal muscles, aided perhaps by that of the 

 adjacent portion of a longitudinal muscle on each side pass- 

 ing from the anterior to the posterior extremity of the 

 prosoma above the nerves radiating to the appendages. 



Does the structure of the most primitive types of euto- 

 sternite known to us furnish justification for the opinion that 

 they have originated in the manner here suggested ? A 

 good deal may be said, i think, in favour of an affirmative 

 answer to this question. 



The points of attachment of the tcrgo-sternal muscles of 

 the opisthosoma are generally apparent enough externally, 

 both on its dorsal and ventral walls. On the prosoma they 

 are usually much less apparent. In the Mygalomorphous 

 spiders, however, the sternum is typically marked with four 

 pairs of " scars," the so-called " sigilla," one on each side 

 close to the proximal end of the coxa of the second, third, 

 ftnirth, and fifth appendages (i. c. the palpi and first three 

 pairs of legs). The position and nature of these scars at 



liciitiou of U)vn\ anil spucialisaliou of boiiy regions entailed a corresponding 

 luodificalioii of the muscular system. Tlie dorsal and ventral longiludinal 

 muscles of the prosoma were suppressed. Tlie muscles of the prosomatic 

 limbs acquired large size a;)d l)ccame subdivided." 



