GEPHYREA. ACII.ETA. 



393 



The cerebral ganglion, cesophageal ring and ventral cord run inside 

 the dermal muscular tunic. Only one pair of segmental organs, 

 known as brown tubes or ventral glands, 

 is present. The blood vascular system is 

 well developed. 



Development. The segmentation is com- 

 plete and is followed by the formation of a 

 gastrula by invagination. The blastopore 

 marks the ventral side. The two posterior 

 marginal cells* of the entoderm move in- 

 wards as primitive mesoderm cells, and give 

 rise to the mesoblastic bands which do not 

 undergo segmentation. Invaginations of the 

 ectoderm of the animal pole and ventral sur- 

 face of 

 the em- 

 bryo give 

 rise to 

 cepha- 

 lic and 

 ventral 

 p late s 

 respec- 

 tively, 

 while the 

 remain- 

 der o f 

 the ecto- 

 d e r m 

 cells 



grow round thes-e and form an 

 external envelope for the embryo 

 of the nature of a serous mem- 

 brane (serosa). Cilia project from 

 the latter through the pores of 

 the vitelline membrane and are 

 employed by the embryo in 



FIG. 317. Quite young Si- 

 piincnliiK still without ten- 

 tacles (after B. Hatschek). 

 O, mouth ; A, anus ; US, 

 ventral cord ; JV, nephri- 

 dhim (brown tube) ; <?, 

 ceiebral ganglion; B<j, 

 blood vessel. 



swimming. 



FIG. 318. Larva of Sipunculua (after Hats- 

 chek). 0, Mouth ; Sj>, apical plate ; A, anus ; 

 PoW, postoral circle of cilia; N, kidney. 



The cephalic and ventral plates 



soon grow together. The mesodermal bands split into somatic and 

 * Compare especially B. Hatschek. 



