42 BRYOZOA BCTOPROCTA. 



In the Phylactolaemata the posterior ends of the semicircular 

 rudiment of the lophophore grow out as large finger-like structures. 

 (Fig. 23, C, I), projecting into the atrium. These are the rudiments 

 of the two arms of the lophophore. The cavities within these 

 processes (lophophore-cavities) are to be regarded as a part of the 

 body-cavity. They communicate with one another through the 

 semicircular canal that encircles the oesophagus. On the anal side, 

 according to BRAEM, they are connected by the so-called forked 

 canal, so that here also the system of lophophore-cavities forms a 

 ring surrounding the oesophagus. Another significance has been 

 attributed to this forked canal by CORI (p. 56). In the Phylacto- 

 laemata the outer tentacles make their appearance before those on 

 the inner side of the lophophore. The former appear first in the 

 neighbourhood of the mouth and gradually fresh tentacles are added, 

 each new one being nearer the free end of the lophophore, those at 

 the apex forming last. The tentacles of the inner row develop in 

 the reverse order, i.e., those nearest the apex arise first, whilst 

 those near the epistome and above the forked canal are the last to 

 appear. * 



The epistome arises as a projecting fold on the anal side of the 

 oral aperture, and into it is continued an outgrowth of the body- 

 cavity the epistomal cavity. It should be mentioned that several 

 authors (SEELIGER and others) maintain that a rudimentary epistome 

 is to be found in the Gymnolaemata. 



The outer or mesodermal layer of the polypide-rudiment gives rise 

 to the following parts : the peritoneal covering of the enteric canal, 

 the muscle-layer of the intestine, and some of the body-muscles, 

 especially the retractors. The development of the latter has been 

 studied chiefly by BRAEM and DAVENPORT. Groups of mesoderm- 

 cells become detached at the neck of the bud, and become inserted 

 at one end upon the polypide and at the other on the zooecial 

 wall. These rudiments of the retractor muscles are originally 

 inserted at a point on the zooecial wall quite near the neck of 

 the bud, and only at a later stage, as the wall develops, do the 

 points of insertion shift further from the aperture of the polypide- 

 rudiment. 



In the Phylactolaemata, the outer mesoderm-layer of the polypide- 

 bud also takes an important part in the development of the funiculus, 

 which will be dealt with later (p. 50). The nephridia of the Phylac- 

 tolaemata also, the existence of which has been maintained by 

 VERWORN and CORI, and recently confirmed by BLOCHMANN, although 



