TYPE PROSOPYGIA. 273 



later elongating laterally to form the coiled lophophore with 

 its numerous tentacles. 



The early stages of the development of the Ecardines is 

 not known, but in Lingula the larva is free-swimming long 

 after the shell has formed, the peduncle being late in develop- 

 ing. In this form also the lophophore arises as a circle of 

 tentacles surrounding the mouth and subsequently elongates 

 laterally. 



The affinities of the Brachiopods have long been an open question. 

 They were by early writers regarded as Mollusca, later as Annelida or 

 closely related to that group, but are now usually considered to be more 

 nearly related to the Polyzoa than to any other forms and to be most 

 properly associated with them, the general likeness of a young Lingula, for 

 instance, to a Polyzoan being very striking. The presence of the mantle- 

 lobes and the shell seem to mark the Brachiopoda as something far removed 

 from the other members of the type Prosopygia, but it must be remembered 

 that in the larval Ectoproctous Polyzoa the corona behaves in a manner 

 closely similar to the Brachiopod mantle and it is not impossible that the 

 two structures may have something in common. 



Another distinguishing feature of the Brachiopods is the indication of 

 a segmentation. The presence of two dissepiments and in Rhynchonelta of 

 two pairs of nephridia certainly suggests metamerism, but objection has 

 been raised to the dissepiments having any metameric significance, on the 

 ground that they do not bear the proper relationships to the body axis to 

 be regarded as comparable to the dissepiments of the Annelida. It has 

 been stated by some authors as a characteristic of the Prosopygia that 

 their body axis is bent upon itself so that the two ends are approximated 

 and one surface, the dorsal, is almost obliterated, while the other, the 

 ventral, is very much enlarged, as seems to be actually the case in 

 Phoronis. It must be remembered, however, that the terms dorsal and 

 ventral are not to be defined by reference to the digestive tract alone, but 

 other structures have also to be taken into consideration. Thus it is quite 

 possible that in the Polyzoa the approximation of the mouth and anus indi- 

 cates simply a bending of the digestive tract and a migration forwards of 

 the anus and not necessarily a bending of the body axis; and the varying 

 position of the anus in the Ecardinate Brachiopods tends to support this 

 idea, the body axis in Crania with a terminal anus certainly being similar 

 to that of Lingula, in which the anus lies far forwards. In this connec- 

 tion, too, the arrangement in Sipunculus is of interest, the nerve-cord show- 

 ing the usual relations to the body axis, while the digestive tract is bent 

 upon itself and the anus opens far in front of the posterior extremity. In 

 the Sipunculacea there can be no question of a difference of the body axis 

 in the various forms, and it seems probable that the supposed bending of 

 the body axis in the Prosopygia has not really occurred, but that there 



