304 



GENERAL SUMMARY TO 



the other cavity, by a second layer of small, pale-coloured hexagonal cells, which for 



some distance are in direct relation with the external layer. (Kowalevsky is unable to 



explain either the origin or signification of this layer.) 



Afterwards the median cavity transforms itself into a digestive tube ; perhaps these 



walls form the intestino-glandular layer. The lateral cavities constitute the general cavity 



of the body ; the interior part of their walls produces the mesentery and perhaps some 



muscular fibres (intestino-muscular layer of the middle layer) ; the outer part gives birth 



to the principal muscles. 



Next follows a stage wherein the embryo is divided into three segments (Eig. C) ; the 



last appearing segment being the median segment or thoracic segment, which has probably 



been formed by a division of the caudal segment. 



The thoracic segment carries four bundles of bristles, of which two are median and 

 two lateral ; each bundle is composed of four or five 

 immovable cylindrical and blunt bristles (Eig. C, p. 

 303). 



At the succeeding stage the mantle commences to 

 appear under the form of two folds covered with 

 hairs, which rise from the ventral and dorsal side of 

 the thoracic segment. 



Soon this folding of the skin (future mantle) 

 becomes developed and partly covers the caudal 

 segment. The bristles, which are then situated 

 on this fold, become larger and pass beyond the 

 caudal segment. At the same time appear two pig- 

 mentary spots on the dorsal side of the cephalic 

 segment. 



The author has not been able to observe the 

 phases intermediate between this stage and that 

 where the larva issues from the incubatory pouch 

 and swims freely. At this period of the larval deve- 

 lopment, the mantle covers all the caudal seg- 

 ment, whilst the cephalic segment, which is covered 



Advanced stage sof larva of Argiope Neapoiitana with vibratile cilia, assumes an umbrella-shape, at 



(swimming freely). l 



^.eyes; r, edge of the cephalic segment ; «. the summit of which may be distinguished a portion 

 SSiATTL&rt^SbLfS more or less se P arated from the rest (head), and which 



the intestine; e, mesentery; m' , a part of the carries four eyes {y, Fig. D) 



this head corresponds 

 Lacaze-Duthiers has described in 



mantle covering the caudal segment ; a, muscle 



going from the dorsal to the ventral part of the to that which M. 



valve ; I, abdominal muscles ; c, muscles of the 



cephalic segment ; s, bristles. Thecidium. 



The digestive tube is almost entirely confined to the thoracic segment ; its anterior 

 portion, however, penetrates a little into the cephalic segment. This last connects itself 



