6 



indicated (text figure 2) very early by the appearance of a pit, the beginning of 

 the shell gland, around which the mantle ridge forms. The oral invagination and 

 the anlage of the cephalic ganglion mark the anterior end of the embryo. The 

 large head-folds, partly surrounding the optic pits, form the greater portion of 

 the sides of the embryo. The statocyst, the anlagen of the siphon, and of the 

 gills lie on the posterior surface. The foot is represented by a band of tissue 

 which connects the embryo with the yolk and from which five pairs of elevations, 

 the anlagen of the five pairs of arms, project. The anus has not yet formed 



but its future position is indicated 



by the mesenteron which lies 



beneath the ectoderm of the back 



of the embryo. The essential relations 



to be noted are , that the foot is 



ventral . the mouth anterior . and 



the shell-gland dorsal. A more 



advanced embryo is represented by 



text figure 3. The convex visceral 



~ ^ ^ dome of the younger embryo has 



Gi become conical and at the same 



41.1 i^, '" time the ventral portion of the embryo 



has contracted constricting the yolk- 



PK t?rj w\... sac from the embryo. The mantle 



has grown downward so that it now 

 covers the gills and the newly 

 formed anus. The gut is complete 

 and near its middle, makes an angle 

 from wiiich two evaginations, the 

 undeveloped stomach and caecum , 

 project upward. The heart is dorsal 

 to the gut. The statocysts , two 



Pn.. 



S(. 



IS 



Ca 



.Ne 



SiV 



A* 



Fip. 3. 



Fi^. 4. 



pairs of siphonal folds , the pedal , and the visceral ganglia are on the posterior 

 side of the embryo between the mantle and the arms. This change may be 

 summed up by saying that the doi'so-ventral axis has elongated while the sagittal 

 and transverse axes have become relatively short. This process continues in the 

 Decapoda beyond the stage represented in text figure 3 until the adult form 

 (text figure 4) is reached. 



In the adult Decapod the dorsal and ventral surfaces are reduced to mere 

 points at the ends of a fusiform body. The reduction of the ventral surface has 



