THE FUSIFORM CELLS. 



447 



the anlagen of the larval appendages and probably such portions of the postoral 

 nerve cords as are represented. 



The preoral lobe corresponds roughly to the labrum and procephalon, and 

 the region covered by the diagonal ciliated band, to the thorax of the naupula. 

 The abdominal region, as in the ectoprocta, is represented by an imaginal disc-like 

 infolding that grows, in a measure, independently of the rest of the larva, C, ab. 

 After a while, the thickened floor of the infolding, that has become irregularly 

 folded, ruptures its amnion-like covering, and is violently everted, carrying a 

 U-shaped fold of the enteron with it. Meantime the anal end of the larva is 

 drawn in a haemal direction, the diagonal band ruptures, the forebrain, preoral 



FIG. 



305. Diagrams illustrating the development and metamorphosis of Phoronis. (In part, after Selys- 



Longschamps . ) 



and marginal lobes break down, and the fragments pass into the enteron through 

 the mouth. The short permanent appendages, which are lined with a prolongation 

 of the coelomic epithelium, assemble in the oral region, forming the basis for the 

 lophophore. 



The primitive cerebral ganglion, or forebrain, disappears with the preoral 

 lobe, and the permanent ganglia appear to be derivatives of the ventral cords, 

 transferred to the haemal surface with the permanent appendages. As in the 

 ectoprocta, these ganglia are connected with each other by a circumoesophageal 

 band that probably represents the remnants of the transverse commissures, which 

 have been greatly elongated by the haemal migration of their appropriate ganglia. 



Fusiform Cells. At about the close of the metamorphosis, many peculiar 

 spindle, or fusiform cells make their appearance, that probably represent the rem- 

 nants of the disintegrating, or the unformed, thoracic musculature. In their 

 histological structure, general appearance and distribution, they agree closely 

 with the fiber cells of Limulus and of many other arthropods. See Chapter 

 XIII, p. 232. 



In Phoronis, they are mingled with the vaso-peritoneal tissue, or float freely 

 in the cavity of the body. They have been regarded as modified blood cells, but 



