296 



LARVAL FORM*. 



usual contractile band, and at the insertion of this band is a thicken- 

 ing of the epiblast which probably represents the rudiment of the 

 supra-cesophageal ganglion. A ventral nerve cord is stated by Salensky 

 to be present, but his observations on this point are not quite satis- 

 factory. 



The metamorphosis is accompanied by the loss of swimming 

 power, and consists in the enlargement of the post-oral portion of 

 the trunk, and in the simultaneous reduction of the pise-oral lobe, 

 which remains however permanently as the cylindrical proboscis. A 

 groove which terminates posteriorly at the mouth is very early formed 

 on its ventral side. The ciliated rings gradually disappear during the 

 metamorphosis. 



Of the further external changes the most important are (1) the 

 early appearance round the anal end of the body of a ring of bristles ; 



and (2) the appearance of a 

 pair of ventral seise in the 

 anterior part of the body. The 

 anterior ring of bristles charac- 

 teristic of the adult Echiurus 

 does not appear till a late 

 period. 



Of the internal changes the 

 earliest is the formation of the 

 anal respiratory sacks. With 

 the growth of the posterior part 

 of the trunk the intestine elon- 

 gates, and becomes coiled. 



Bonellia. The embryo of 

 Bonellia, while still within the 

 egg, retains a spherical form 

 and acquires an equatorial band 

 of cilia, behind which a second 

 narrower band is soon establish- 

 ed, while in front of the first one a pair of eye-spots becomes formed 

 (fig. 162 A). The embryo on becoming hatched rapidly elongates, 

 while at the same time it becomes dorso-ventrally flattened and ac- 

 quires a complete coating of cilia (fig. 162 B). According to Spengel 

 it resembles at this time in its form and habits a rhabdoccelous Tur- 

 bellariari. The anterior part is however somewhat swollen and pre- 

 sents an indication of a prse-oral lobe. 



During the above changes important advances are made in the formation 

 of the organs from the embryonic layers. 



The epiblast acquires a superficial cuticula, which is perhaps directly 

 derived from the vitelline membrane. The nervous system is also formed, 

 probably from the epiblast. The band-like supra-cesophageal ganglion 

 is the first part of the nervous system formed, and appears to be un- 

 doubtedly derived from the epiblast. The ventral cord arises somewhat 

 later, but the first stages in its development have not been satisfactorily 



FIG. 161. LAEVA OF ECHIURUS. 

 (After Salensky.) 



m. mouth; an. anus; Kg. snpra-ceso- 

 phageal ganglion (?). 



