532 



ANNELIDA. 



a delicate postoral circle (Poic). In Ediiurus early in larval life an 

 excretory organ, the head-kidney or pronephros (KN) is developed, 

 one on either side ; and behind it a pair of mesoblastic bands make 

 their appearance, and become divided up in subsequent development 

 into the rudiments of 15 pairs of mesoblastic somites (Fig. 431, &). 

 In the terminal segment, which is surrounded by a circle of cilia, 

 there appear in the somatic mesoderm the rudiments of the anal 

 vesicles (Fig. 431, AS). The rudiments of both the cerebral ganglion 

 and of the ventral cord are derived from growths of the ectoderm 

 the former from the apical plate, and the latter from paired thicken- 

 ings of the ventral ectoderm. The two are connected by the 

 oesophageal ring, which is also provided with ganglion cells. In 

 later stages the segments disappear, and the ciliary apparatus 

 degenerates and vanishes; after which two strong hooked setae 



appear at the side of the nerve cord, not 

 far from the mouth, and two circles of 

 shorter setae are formed at the hind end 

 of the body (Fig. 432). The preoral lobe 

 of the larva becomes the proboscis of the 

 young Ediiurus. In Thalassema the 

 development is very similar, and the meso- 

 blastic bands are segmented. In Boneliia, 

 on the other hand, no traces of segmentation 

 have as yet been observed, and the larva is 

 not so obviously built on the trochosphere 

 type. 



In view of the development of EcJiiurus and 

 Thalassema, it is difficult to resist the conclusion 

 that the Echiuroidea are Annelida, a conclusion 

 which is further emphasized by the presence in all 

 genera of typical Annelidan setae embedded in and 

 secreted by pits of the skin. It is curious that in 

 Boneliia the trochosphere stage is so much modified, 

 and especially that no traces of segmented meso- 

 blastic bands should be seen. With regard to the 

 latter point, however, it must not be forgotten 

 that a renewed investigation may yet bring to 

 light some traces of larval segments. We may 

 therefore regard the Echiuroidea as Annelids in 

 which the segmentation is feeble, showing faintly 

 in the young, but except in the repetition of the 

 nephridia (e.g. Thalassema with three pairs of 

 anterior and one pair of posterior uephridia), 

 being absent from the adult. 



AS 



FIG. 432. Older Echiunts larva 

 from the side(afterHatschek). 

 The head-kidney is atrophied. 

 A anus ; AS anal vesicles ; 

 BK circles of setae ; G cere- 

 bral ganglion developed from 

 the apical plate ; H ventral 

 hooks ; M stomach ; mouth ; 

 SC oesophageal commissures; 

 VG ventral nerve cord. 



