306 



GENERAL SUMMARY TO 



over of the mantle the external part of each lobe becomes internal, from which it 

 results that the bristles that were on the exterior surface find themselves carried inside, 

 and then, becoming unnecessary, drop off in a few days' time. It is therefore seen by 

 this, that it is not these bristles which produce those that border the mantle of the adult 

 animal ; anyhow, Argiope is not provided with marginal bristles or setae, but in those 

 species that possess them they are the result of a new formation and of a much later 

 period. 



The lobes of the mantle gradually become invested with a thick and ridged 

 cuticle which permits them to move only in a vertical direction. At the same time the 

 caudal segment is transformed into a peduncle, and the muscles that went from this last 

 segment to the thoracic segment become ventral peduncular muscles, the pair of middle 

 muscles changing into divaricators. The head assumes a spherical shape, and the eyes 

 continue to exist. 



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

 becomes round, and there is produced in the cephalic segment a funnel-shaped pit 

 F which descends towards the canal. This depression 



is in all likelihood the oesophagus (gullet). It should 

 be observed that the characteristic feature at this 

 period consists in the appearance of the branchiae; 

 these come into existence in the form of four nipples 

 directed inwardly and situated on a thickening of the 

 dorsal lobe near its border. 



The dorsal thickening soon assumes the form of 

 an almost circular swollen ring, and the number of 

 the branchiae amounts to ten (b of woodcut F). In 

 the interior of the swollen ring there exists a cutaneous 

 layer, which seems to support all the branchial 

 apparatus, and which is pierced by a hole opposite 

 each of the branchiae. The branchiae, which are 

 only prolongations of the swollen ring, are covered 

 with vibratile cilia, and are provided with flexible 

 muscles and become erected through their own 

 extremities of the branchiae converge towards the opening of the 



Larva of Argiope Neapolitana. 

 Condition previous to the formation of the 

 shell, b, branchiae ; f, muscular bundle of 

 the branchia; of the inner side ; t, head and 

 oesophagus; z,divaricator muscles; i, stomach; 

 mp, ventral muscles of the peduncle ; mi, 

 muscles going from the ventral valve to the 

 upper part of the dorsal valve ; ei, internal 

 envelope ; p, peduncle. 



elasticity. The 

 mouth. 



At the following stage there are present twelve tentacles ; and at the same time the 

 shell is formed, but no tubular perforations are yet seen. Of the following stages 

 Kowalevsky has been able to trace but one, that of the gradual formation of the shell. 



Lacaze-Duthiers' admirable description of the development of Thecidium was a 

 great advance to our knowledge. He showed that the embryo and larva, as in 

 Argiope> pass through different stages previous to reaching the full grown condition. 



