460 Eepoet of the State Geologist. 



When we compare the muscular system of the fresh-water 

 and marine forms we find a great similarity in them, but also 

 some modifications. In all forms the endocyst continues beyond 

 the ectocyst and is continuous with the margin of the lopho- 

 phore. In the marine forms, when the animal is exserted, the 

 endocyst or tunic is, of course, carried with the lophophore, and 

 not being held by any muscles is continuous in essentially a 

 straight line with that portion of the endocyst lining the cell. 

 "When the animal is retracted the endocyst is invaginated, the 

 evagination and invagination being complete; the evagination 

 being effected in the same manner as if the finger of a glove 

 was pulled in from its inner extremity, the invaginated portion 

 forming the tentacular sheath. 



In the fresh-water forms the evagination and invagination is 

 incomplete, a portion of the endocyst being permanently in- 

 vaginated. 



Though the evagination in Paludioella. is not complete, yet it 

 in common with the marine forms, is without the small radiat- 

 ing muscles near the orifice and the larger radiating muscles 

 which are connected with the tentacular sheath (fig. 19, 6>, r). 

 The polyp retractor muscles are essentially the same in all forms 

 of Bryozoa. The sphincter muscles are present in Paltjdicella 

 and the marine forms. The parietal muscles are the same in all 

 forms, and the tube retractors are essentially the same. Paludi- 

 CELLA is very closely related to the marine forms even in its mi- 

 nute structure. 



The protrusion of the animal from its cell takes place as fol- 

 lows : The principal agency to which we must assign the pro- 

 trusion of the animal is undoubtedly the contraction of the endo- 

 cyst by the parietal muscles and the consequent pressure of the 

 perigastric fluid upon the body of the animal, making it neces- 

 sary for it to move in the direction of the least resistance, namely, 

 the orifice of the cell. 



Faere, in his admirable paper on Bowerbankia densa, states 

 that the principal cause of the protrusion of the animal is 

 the pressure of the perigastric fluid, and that the straightening • 

 of the oesophagus raises the tentacular crown a certain distance, 

 which it undoubtedly does, as the oesophagus has the power to 

 straighten itself without the action of the fluid. 



