308 



EMBRYOLOGY 



latter acquire cavities and enlarge in the well-known manner. 

 Just as in the other Annelida, so in the Echiuridce (Fig. 

 145), there is established an internal segmentation, corre- 

 sponding to which there is an outer one, in so far as a 

 large number of segmental ciliated bands make their appear- 

 ance on the posterior part of the Echiurus larva. But this 

 segmentation is only temporary, for, like the bands of cilia, the 

 septa between the cavities of the segments also degenerate. 

 Of the fifteen primitive segments which were begun, only the 

 somatic and splanchnic layers remain, and, as the resnlt of 

 the disappearance of the septa, the secondary body cavity of 



the trunk unites with the 



z—T^' 



■V'' 



sc. 



tn. 



primitive head cavity. Like 

 the cavity of the trunk, the 

 head cavity is also tra- 

 versed by branched cells 

 (Fig. 146), and since these 

 are in part applied to the 

 ectoderm, the dermo-muscu- 

 lar sac, which was estab- 

 lished in the trunk at an 

 earlier period, is also deve- 

 loped in the head region. 



The nervous system is 

 also established in the larva. 

 On either side of the ventral 

 ciliated groove arise thick- 

 enings of the ectoderm, from 

 which small nodular pro- 

 cesses grow inward, and 

 unite segmentally into large 

 masses of cells, the ventral 

 ganglia (Figs. 145 and 146). In this way the lateral cords 

 arise, to which a middle cord is added. The latter sepa- 

 rates from the ectoderm of the ciliated groove. At first 

 the entire ventral cord is still intimately united with the 

 ectoderm, but the latter gradually detaches itself, and the 

 ventral cord thereby acquires a deeper position. The funda- 

 ment of the supra-oesophageal ganglion, which is small in the 



Fig. 145. — Trocliophore larva of Echiimis 

 (after Hatschek). a, anus; ah, anal 

 vesicle ; d, intestine ; fen, head kidney ; m, 

 mouth ; mes, mesodermal bands ; n, ven- 

 tral chain of ganglia; sc, oesophageal con- 

 nective; sp, apical plate. The ciliated 

 bands of the posterior part of the body 

 are indicated by the cilia at the margins 

 only. 



