CHAETOGNATHA 



621 



Sagitta bipunctata is one of the most characteristic and cosmo- 

 pohtan members of the plankton and is a typical pelagic organism 



ms.s. 



\ ' I / I I 



Fig. 431- 



Fig. 432. 



Fig. 431. Phoronis. Altered from Delage and Herouard. Sagittal section 

 to show half the lophophore, the alimentary canal, one of the nephridia 

 and most of the vascular system. The middle part of the section is omitted. 

 An. anus ; ajf.v. afferent and eff.v. efferent vessels ; ep. epistome ; ga. ganglion ; 

 int. intestine ; Iph. lophophore ; Iph.v. lophophoral vessel giving off a branch 

 to each tentacle; nep.a. nephridial aperture; nephr. nephrostome; 7i.r. nerve 

 ring; oik ovary; org. lophophoral organ (paired); sep. septum between two 

 divisions of the coelom ; st. stomach ; ts. testis ; ten.v. tentacular vessel. 



Fig. 432. Actinotrocha and its metamorphosis into the adult. After various 

 authors. A, Actinotrocha larva with ciliated lobes (cil.l.) and rudimentary 

 visceral sac (vis.s.). B, \isceral sac evaginated. C, Growth of visceral sac and 

 decrease in size of preoral lobe (pr.L). D, Metamorphosis : mouth and anus 

 approximate, preoral lobe becomes epistome (ep.), ciliated lobes of larva 

 are seen in the mouth and new tentacles "are beginning to grow. An. anus ; 

 M. mouth. Alimentary' canal shown by stippling, visceral sac by oblique 

 shading. 



with its glassy transparent body and its powers of vertical migration ; 

 off the coast of California it lives at a depth of 15-20 fathoms during 



