MOLLUSCA: BRACHIOPODA. 393 



greater development, and consists of a gangliated oesophageal 

 collar. 



The sexes in the Brachiopoda appear to be ordinarily distinct, but in 

 some forms they are asserted to be united in the same individual. As 

 regards the process of development in the class, we may take as a type 

 Terebratulina septentrionalis, the metamorphoses of which have been most 

 ably worked out by Professor Morse. In this form, the earliest embryo is 

 a ciliated planula (fig. 205, A), which swims about actively, and soon (B) 

 exhibits a division into three regions or segments, which rapidly become 

 more conspicuous (C). Of these segments, the most inferior (/) becomes the 

 future peduncle, and serves to attach the embryo to some foreign body (D). 

 The middle segment then enlarges, and partially encloses the anterior seg- 

 ment (E and F), the latter ultimately being withdrawn entirely within the 

 former, which becomes converted into the shell-secreting pallial lobes. Next 

 the arms begin to bud out of the sides of the mouth (G), forming at first 

 a circular crown of cirri (c), which forcibly calls to mind the orbicular 

 lophophore of the Gymnolsematous Polyzoa. The peduncle, at first long 

 (as in Lingula\ becomes rapidly shorter (I), and the oral crown of tentacles 

 becomes distinctly horse-shoe-shaped (J), thus strikingly resembling the 

 similarly-shaped lophophore of the ' ' Hippocrepian " Polyzoa, The cir- 

 rated " arms " of the adult are finally produced by the growth and devel- 

 opment of the free end of the horse-shoe. 



AFFINITIES OF THE BRACHIOPODA. 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 form- 

 ing 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 Brachio- 

 pods 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 resem- 

 blance 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. 



DIVISIONS OF THE BRACHIOPODA. The Brachiopoda may be divided 

 into the two orders of the Inarticulata (or Tretenterata) and the Artictilata 

 (or Clistenterata). . 



