44Q 



GEPHYREA 



up into a series of somites or segments. Of these there are 

 fifteen in Echiurus, and apparently eleven in 

 Th. mellita ; in this stage the Gephyrean 

 larvae have again so close a resemblance to 

 the segmenting Chaetopocl larvae as to be 

 easily mistaken for them. The segmentation 

 is shown in the following way : (i.) the 

 middle layer of cells or mesoblast is typically 

 segmented, and forms septa, which separate 

 each segment from its neighbours ; (ii.) the 

 ventral nerve - cord arises as segmentally- 

 arranged thickenings of the epiblast, which 

 fuse together, but retain their segmented 

 appearance for some time ; (iii.) the skin shows 

 the segmentation of the body both by the 

 arrangement of the pigment and by bands of 

 cilia. The latter are replaced in the adult by 

 rows of spines, and on the fourteenth and 

 fifteenth segments in Echiurus pallasii by 

 the two peri-anal circles of bristles. Each 



FlG - s bristle, like those of Chaetopods, originates from 



pallasii Guer. x \. r 



a, Mouth at the end a single cell. 



Dosc^^TTenS The aiial vesicles arise q uite l ^ in ^ 



hooks ; c, anus. development ; when they have acquired their 

 openings into the body -cavity, they seem 

 to take in water. In Thalassemia, as described by Conn, this 

 is accompanied by remarkable changes, amounting almost to 

 a metamorphosis. The body increases in bulk fourfold, the 

 cilia of the prae-oral ring disappear, and the animal now moves 

 only by means of its proboscis ; the pigment is absorbed, and all 

 traces of segmentation disappear. A similar intaking of water 

 is described by Spengel in Bonellia. In this genus the larva, 

 which is coloured bright green, and has two brown eye-spots, is 

 not such a typical Trochosphere as is that of Echiurus and 

 Thalassema. 



Nothing is known of the development of Hamingia or of 

 Saccosoma. 



Species of Echiuroidea. Echiurus. Proboscis not bifurcated 

 at the end. Two ventral hooks and a single or double peri- 

 anal ring of bristles. The body is to a varying extent marked 



