584 



BRACHIOPODA. 



and that there is no account of the formation of the structures which are 

 supposed to be transverse septa in the adult, or of the nephridia. 



But this Annelid view of the group is not the only one which has been put 

 forward. It has been suggested that the arms, with their tentacles, are com- 

 parable to the lophophore, and the lip to the epistome 

 of Phoronis (Fig. 465). There is much to be said for the 

 affinities with Phoronis which this view suggests, but it 

 must be remembered that the view is one which itself 

 rests upon a basis of hypothesis (see p. 583), for in neither 

 group has the development of the lip been satisfactorily 

 followed. 



The most important objection to it is that the flexure 

 of the intestine, and the position of the anus nearer the 

 ventral valve, is different to the arrangement in Phoronis, 

 in which the intestine has the opposite flexure, and the 

 anus is on the epistome side (dorsal) of the mouth. It 

 may fairly be urged, however, that, in our present state 

 of ignorance of the development of the Brachiopods with 

 an anus, and consequently of the real position of that 

 organ, too much weight must not be attached to this 

 objection. The possible presence of three mesoblastic 

 segments, and the relations of the nephridia and coelom, 

 are not opposed to it, nor is the mode of attachment by 

 the caudal region ; for it is argued the caudal region 

 might fairly be regarded as equivalent to the evaginated 

 foot of Phoronis. We think the suggestion that the 

 Brachiopods are allied to Phoronis an attractive, but at 

 the same time a highly speculative one. For the principal 

 arguments in its favour, viz., that in both the preoral 

 lobe shrivels up or disappears, leaving at most the epis- 

 tome as an indication of its presence, and that in both 

 the dorsal surface between the mouth and anus is extra- 

 ordinarily shortened, are open to the serious reply, that 

 although these statements may fairly be made of Phoronis, 

 they can only be put forward in a hypothetical form with 

 regard to Brachiopods. 



FIG. 467. Longitudi- 

 nal section of an 

 older larva of Lin- 

 gula (after Brooks). 

 Do dorsal ; Ve ventral 

 valve of shell ; Mr 

 thickened edge of 

 mantle; T tentacles; 

 mouth ; Md stom- 

 ach ; Ad intestine ; 

 M posterior muscle ; 

 G ganglion. 



The Brachiopoda are found in all seas at different depths. The 

 larger number live at moderate depths, down to 500 or 600 fathoms. 

 Beyond this depth they are rare, though species of Discina and 

 Terebratula have been taken at a depth of over 2000 fathoms. 

 Lingula and Glottidia are found between tide-marks. For the most 

 part they live on rocky ground, and are found in great numbers 

 together. Lingula burrows in the sand. 



The number of living Brachiopods is small, as compared with the 

 much larger number found in the earlier geological formations, 

 certain species of which have great importance as characteristic 

 fossils. The oldest known fossils are Brachiopods, and certain 



