Great differences of opinion exist at the present day as to the affinities and precise systematic position of the Brachiopoda; but it is impossible to do more here than merely point out these differences. The relationship of the Brachiopods to the Polyzoa is admitted on all hands to be very close; and we may regard the encrusting members of the latter class as being "communities of Brachiopods, the valves of which are continuous and soldered together, the flat valve forming the united floor, whilst the convex valve does not cover the ventral valve, but leaves an opening more or less ornamented for the extension of the lophophore " (A. Agassiz). Until recently, most naturalists have held that both these groups had strongly-marked relationships with the Lamellibranchiata, and many still adhere to this view. On the other hand, the view has been gaining ground, that these groups are to be regarded as comprising modified worms, and they are often placed in the immediate neighbourhood of the Annelida. The chief grounds for this view are to be found in the similarity of the development of the Polyzoans and Brachiopods to that of the Annelides, as shewn by the elaborate researches of Morse and Kowalewsky. Apart from embryological likeness, one of the most striking links between the Brachiopods and the Annelides is the aberrant Lingula pyramidata - the genus Lingula being itself an aberrant type. This curious form (fig. 203, A), as described by Morse, differs from its congeners in not being fixed, but in living free in the sand. Its peduncle is long and wormlike, hollow, and highly contractile, and its lower end is encased in a sand-tube, resembling that of a Tubicolous An-nelide. Whilst it must be freely admitted that the affinities between the Brachiopoda and the Annelides are much closer than any outward resemblance between the two would lead us to expect, a sufficient case for the removal of the former from the Mollusca has hardly been made out, except in the view of those who place a supreme value upon embryological characters in classification.