DR. T. DAVIDSON OX EECEXT BEACHIOPODA. 



135 



is thus seen that it is not these hristles which produce those that horder the mantle 

 of the adult animal. Argiope, it is true, is not provided with marginal bristles or 

 setae, but in species wherein they occur they are the result of a new formation of a 



Pis. 1-'. 



£z^n. 



Argiope {CisicUa) nerqjolitanu (after Kowalevskv). 



Immediately after it lias become fixed. (The tlioraeio segment lias four bundles of bristles, of which two are 

 median, and two lateral.) s, bristles ; ij, ej-es ; m, muscles going to the basis of the bristles ; md, adductor 

 muscles ; 7np, ventral peduncular muscles. 



much later period. The lobes of the mantle gradually become invested with a thick 

 and ridged cuticle, which permits theni to move only in a vertical direction. At tiie 

 same time the caudal segment is transformed into a peduncle, and the muscles that 

 proceeded from this last segment to the thoracic segment become ventral peduncular 

 muscles, the middle pair of muscles changing into divaricators. The head assumes 

 a spherical shape, and the eyes remain. 



At the next stage the thoracic segment grows smaller in size, the digestive canal 

 becomes round, and a funnel-shaped pit which descends towards the canal is produced 

 on the cephalic segment. This depression is, in all likelihood, the oesophagus 

 (gullet). It should be observed that the characteristic feature of this period consists 

 in the appearance of the brancliias ; these come into existence in the shape of four 

 nipples directed inwardly and situated on the thickening, of the dorsal lobe near its 

 border. 



The dorsal thickening soon assumes the form of an almost circular swollen ring, and 



