Conditions in the Arthropoda 



61 



that the plane of the gastrula mouth is the plane of the future 

 ventral surface. For instance in the Copepod Cetochilus the 

 blastopore is a longitudinal slit on the future ventral surface and 

 it closes from before backwards, the part which closes last being 



Fig. 33. A, B, Podarke, after Treadwell; C, D, Hydroides 



uncinatus, after Shearer. 



Semi-diagrammatic views to show the derivation of mouth and anus openings, 

 an anus; mes ectomesoblast ; ewendoderm; M e coelomesoblast ; storastomo- 

 daeum; m mouth; hk head kidney; st stomach; av anal vesicle. 



near the site of the future proctodaeal opening. In Limulus 

 and the scorpion or in insects such as Doryphora there is a median 

 ventral furrow which represents the blastopore; and above all 

 there is the evidence of the intermediate form Peripatus, where 

 the blastopore actually elongates into a long narrow slit which 



