Geneea of the E'orth American Palaeozoic Bryozoa. 455 



walls of the cell and the buds which take their origin in the 

 endocyst, and must be nourished by it until they have acquired 

 such a stage in their growth that they have an external orifice. 



The Muscular System. 



The following description of the muscular system refers espec- 

 ially to that of a fresh-water species, Paludicella Ehrenbergi; 

 but it is almost identical with that of the marine forms, and 

 mention will be made of the details in which it differs. 



There are six sets of muscles, three in connection with retrac- 

 tion, two with protrusion and one for the closing of the orifice 

 when the animal has withdrawn in its cell. 



Of the retractors one set acts directly upon the orifice, the 

 others upon the tubular orifice of the cell. The former is much 

 the more powerful muscle (Plate A, V). It takes its origin in the 

 endocyst, toward the bottom of the cell, and passing upward, 

 alongside of the alimentary canal, its superior extremities are 

 inserted around the base of the lophophore. It is composed of 

 numerous long, stout fibers, which by their action draw the ani- 

 mal into the cell. In the marine and in the Hippocrepian forms 

 the corresponding retractor muscles differ only in being com- 

 posed of two fascicles instead of only one. 



In the withdrawal of the animal into the cell, the tentacula, 

 which when well expanded are in the form of an inverted cone, 

 are first brought together in a straight line, and immediately 

 begin to descend. The body does not descend in a mass, but 

 becomes folded up so as to be accommodated in the cell. The 

 oesophagus, surmounted by the tentacles, descends first ; whilst 

 the integument of the upper part of the animal begins to be 

 inverted, where it has its insertion around the tentacular disc. 

 As the tentacles descend the invagination of the integument 

 continues, forming a close sheath around the tentacles. The 

 withdrawal of the animal continues until the extremities of the 

 tentacles ha^re reached a point level with the top of the ectocyst ; 

 that is, to the unyielding part of the cell. The stomach of the 

 animal is now near the bottom of the cell ; the oesophagus is 

 bent in the form of a letter S ; the tentacles are lying close 

 together inclosed in their tegumentary sheath. It now remains 

 to complete the closing of the orifice. This is effected by the 



