Geneea of the ITorth American Palaeozoic Bryozoa. 461 



As the parietal muscles contract, the endocyst leaves the 

 ectocyst, especially in the lower part of the cell, and the animal 

 begins, on account of the consequent compression of the perigas- 

 tric fluid, to move up the cell. ' The sphincter muscles relaxing 

 affords to the tentacles an easy passage through the inverted 

 endocyst. The tube retractors relaxing the inverted lips of the 

 orifice begin to evolve. As the contraction of the parietal 

 muscles continues, the protrusion goes on until it is complete. 

 In the marine forms the whole of the invaginated endocyst is 

 protruded ; in the fresh- water forms with the single exception of 

 Paludicella, nearly all that portion of the endocyst continuing 

 beyond the ectocyst is permanently invaginated, its evagination 

 being stopped by the radial muscles (fig. 18, o and fig. 24, <?.) 



In Paludicella there is a more complicated process. The re- 

 laxation of the tube retractors permits the endocyst to be evagi- 

 nated, but only for a short distance, the posterior fibers of these 

 muscles soon checking its further progress, and keeping a small 

 portion of the endocyst permanently invaginated. The second 

 portion of the endocyst, which when invaginated constitutes the 

 tentacular sheath, continues to be evolved by the relaxation of the 

 posterior parietal vaginal muscles, but the relaxation, soon ceasing 

 the complete protrusion is prevented. Thus we have two small, 

 permanent invaginations after the eversion is completed as 

 shown in Plate A, fig. 1, one within the other, forming the mem- 

 branous cup at the mouth of the cell. The retraction can be 

 much more easily understood. The parietal muscles relax ; the 

 polyp retractor contracts, pulling the animal in the cell. The 

 superior tube retractors first, and then the inferior contract, and 

 when the animal is withdrawn entirely within the cell, the action 

 of the sphincter muscles closes the orifice. When an operculum 

 exists this is closed by the action of the muscles previously 

 described. 



The Nervous System. 



The nervous system consists of a single ganglion attached to 

 the external surface of the oesophagus (fig. 1, ^, and fig. 22,/), 

 with its filaments. 



In the Hippocrepian forms filaments have been distinctly 

 traced by Allman running to the tentacular crown. The 

 ganglion gives off a rather thick cord, immediately running into 



