ARTICULATA AND MOLLUSCA. 215 



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ticulata as a whole, if not above, at least fully on a par with 

 the Mollusca. 



There is a remarkable correspondence to each other in the 

 respectively repeated parts ; for example, the nerve ganglions or 

 knots correspond to the joints of the body ; so do the successive 

 chambers of the heart, and the breathing apertures, stigmata^ or 

 gills, which open along the sides of the joints. This is especially 

 noticeable in the lower kinds of Articulata ; but as we ascend to- 

 ward the more elevated groups, these repetitions are subjected to 

 the same systematic reduction as we have seen operating among 

 Zoophytes. 



Of all the Articulata, perhaps none are so lowly organized, 

 and yet at the same time exemplify the typical idea of this di- 

 vision so fully, as the Tape-worms. These I have already given 

 an account of, (p. 83,) and I need, therefore, merely to recur 

 to them here for the sake of bringing their characters into place 

 at the lowest point in the successively rising scale of rank. 

 The much more highly organized Myrianida, which I have also 

 described, (p. 80,) will serve as an example of the extent to 

 which the marine worms carry out the idea of serial repetitions, 

 and through a much greater range than Tsenia, in fact to the high- 

 est degree that is known among Articulata. It might seem to 

 you that on this account Myrianida should rank below Tsenia ; 

 but the latter is far inferior to the former simply because it almost 

 totally lacks some of the organs which are so highly developed 

 in the marine worm, and therefore the diversity and degree of 

 specialization being also less than in the Myrianida, it of course 

 stands lower in the scale. 



Bonellia. The most decided step toward that uniformity of 

 organization which culminates in the highest orders of Insects is 

 taken by a group of worms which, curiously enough, until of late 

 years has been classed by almost all naturalists among the Tre- 

 pangs, one of the classes of the grand division of Zoophytes. 

 To this group belongs the worm whose organization I have illus- 

 trated by these diagrams (figs. 126, 127). There is scarcely a 

 trace, only a mere rudiment, of the external locomotive append- 



