HYATT, OBSERVATIOiSrS ON POLYZOA, 145 



VIII. Observations on Polyzoa. Suborder PhylactolcBmatcu 

 By Alpheus Hyatt, 



[Continued from page 112.] 



(4) The fourth, or epithelial layer envelopes the 

 canal, as a Avhole, but is not adherent to the exterior. It 

 passes in a straight line over the depression betAveen the 

 oesophagus and stomach, and is carried from the cesopha- 

 gus to the intestine with but a slight anterior flexure 

 (PL 10, fig. 1, PI. 11, fig. 1). This adaptability of the 

 fourth layer shows that it is accompanied by the same 

 longitudinal muscular fibres which were observed to be as- 

 sociated with it in the endocyst of the polypide, and in 

 the coenoecium. 



During the retraction of the polypide of Pectin atella 

 the alimentary canal is bent upon itself towards the ab- 

 dominal side, and the fourth layer contracts, so that when 

 viewed laterally it appears in a straight line spanning the 

 space between the lower portion of the oesophagus and the 

 end of the stomach (PI. 9, fig. 13, J""). 



AUman defines three layers in the stomach; (1) an 

 internal layer of easily separable spherical cells, filled with 

 colorless fluids, in which float secondary cells with yellow- 

 ish brown contents ; (2) a more compact layer of smaller 

 simple cells with colorless contents and brilliant nuclei ; 

 and (3) a thin membrane of an undoubtedly cellular 

 structure. Of this last he says, "Delicate circular strias 

 may generally be distinctly observed in it ; they may be 

 seen surrounding the stomach, and are probably muscular 

 fibres ; they become less distinct as we ascend towards the 

 oesophagus, and totally disappear from this tube and from 

 the rectum." 



The same author describes but two oesophagal mem- 

 branes, which he considers the equivalents of the middle 

 and external layers of the stomach. Although greatly 

 advancing our knowledge of the tissues, this experienced 

 observer did not see the fourth membrane, and failed in 

 tracing his first membrane, equivalent to my first two, in 

 the oesophagus, where, according to the description given 

 above, it is quite as distinct as in the stomach, cliffer- 



COMMUNICATIONS ESSEX INSTITUTE, VOL. V. 20 AUG., 1867. 



