304 THE INVERTEBRATA 



they should come into contact with the body of the adult female, she 

 exercises (probably through the action of some specific secretion) a 

 largely repressive effect on further development, but a male gonad 

 is formed. 



Thalassema. The British representative, T. neptuni, has a single pair 

 of segmental organs. In two Japanese species, T. taenioides and T. 

 misakiensis, these have been greatly multiplied so that in the former 

 there are 200 pairs rather irregularly arranged. From a consideration 

 of these forms it appears that the multiplication of the segmental 

 organs is a secondary phenomenon. 



Class SIPUNCULOIDEA 



Annelids with a spacious uninterrupted coelomic cavity and few signs 

 of segmentation: without prostomium in adult; chaetae always ab- 

 sent, anterior part of body invaginable into posterior part ; anus dorsal 

 and anterior ; with a single pair of segmental organs (brown tubes) ; 

 in Phascolosoma a trochosphere with three pairs of mesoblastic somites 

 which soon disappear. 



Sipunculus (Fig. 209 C) and Phascolosoma are British genera. 



