ECHIUROIDEA DEVELOPMENT 439 



contains circular and longitudinal muscle-fibres. The body-cavity 

 is developed, but does not reach to either end of the body. The 

 alimentary canal is closed, neither mouth nor anus existing ; it 

 is supported by regularly arranged dorso- ventral muscle strands. 

 A nerve-ring and a ventral cord exist. There are also two 

 rudimentary organs corresponding with the anal vesicles of the 

 female, and a single nephridium which acts as a duct for the 

 spermatozoa ; the latter arise from modified cells lining the 

 body-cavity. 



In both sexes the larvae develop to a certain stage without 

 showing any trace of sexual differentiation, but after this stage, 

 the development of the male is to a certain extent arrested ; in 

 some respects, indeed, it undergoes retrogressive changes. At 

 this time it is found clinging to the proboscis of the female, 

 thence it makes its way to the mouth, where it undergoes its 

 final change ; and then creeping out, finds its way into the nephri- 

 dium of the female, and spends the rest of its life there in 

 a special recess cut off by a fold from the excretory part of this 

 organ. In Hamingia, however, Lankester, who first described 

 the male, did not find any in the nephridia, but found five 

 specimens, each ^ inch long, within the dilated pharynx of 

 the female. 



Development.- In Bonellia and Hamingia it seems probable 

 that the ova are fertilised in the nephridium of the female ; in 

 the other genera they are fertilised in the water after leaving the 

 body of the mother. 



In Thalassema and Echiurus the growth of the embryo results 

 in the formation of a typical Trochosphere larva, a type widely 

 spread in the animal kingdom, being found in the Chaetopoda 

 (Fig. 145, A), Polyzoa (p. 510), and Mollusca. The large prae- 

 oral lobe persists in the Echiuroidea as the proboscis ; the mouth 

 is ventral in position, with usually a ring of cilia encircling 

 the body in front of and behind it ; the anus is posterior and 

 terminal. A pair of larval excretory organs are present, and a 

 special nervous aggregation of cells at the apex of the prae-oral 

 lobe is usually indicated by the presence of a bunch of long 

 cilia. 



The trunk of the Trochosphere is unsegmented, and in 

 certain groups of animals it remains so, but in Chaetopods, and 

 in Echiurus and Thalassema, it elongates and becomes divided 



