ARTICULATA. 



of long cilia, while elsewhere it is provided with a coating of short 

 cilia. Two pairs of eyes also arise on its anterior surface (fig. 136). 



After swimming about for some time the larva becomes fixed 

 by its hind lobe, and becomes gradually transformed into the 

 adult. The hind lobe itself becomes the peduncle. After 

 attachment the mantle lobes bend forward (fig. 137 A, m), and 

 enclose the ce- 

 phalic lobe. 

 The valves of 

 the shell are 

 formed on their 

 outer surface as 

 two delicate 

 chitinous plates 

 (fig. 137 B). At 

 a somewhat la- 

 ter stage the 

 provisional bri- 

 stles are thrown 

 off, and are 

 eventually re- 

 placed by per- 

 manent setae 

 round the edge 

 of the mantle. 

 The cephalic 

 lobe becomes 

 located in the 

 dorsal valve of 

 the shell, and 

 the mouth is 

 formed near the 

 apex of the ce- 

 phalic lobe im- 

 mediately ven- 

 tral to the eye-spots, by an epiblastic invagination. The per- 

 manent muscles are formed out of the muscles already present 

 in the embryo. 



Around the mouth there arises a ring of tentacles, very 



FlG. 137. TWO STAGES IN THE DEVELOPMENT OF 

 ARGIOPE, SHEWING THE FOLDS OF THE MANTLE GROWING 

 OVER THE CEPHALIC LOBE. (After Kowalevsky.) 



m. mantle fold ; me. mesenteron ; pd. peduncle ; b. pro- 

 visional setae. 



