OBSERVATIONS ON POLYZOA. 149 



nal structures. The intimate composition of the circula- 

 tory fluid is not accurately known, although supposed by 

 Prof. Allman to consist mainly of water charged with the 

 chyliferous exudations of the alimentary canal. This is 

 the most reasonable view, though it is not known by what 

 means the water enters the coenoacial cavity. Prof. All- 

 man observed, that the cosncecium of Lophopus and Cris- 

 tatella readily emptied themselves when taken out of 

 water, and I have observed the same phenomenon in Plu- 

 rnatella vitra, which like Lophopus has a soft ectocyst. 



The ccenoecia of these species, when exposed for any 

 length of time, shrink to an almost imperceptible film, but 

 upon being again returned to the water they expand to 

 their former dimensions. The dessication must have been 

 occasioned by the flow of fluid through the pores of the 

 co3ncecial endocyst and the superincumbent ectocyst, since 

 all the polypides were closely retracted. These facts 

 show, that it is not necessary to look for an explanation 

 of the admission of the water to any specialized orifices in 

 the evaginable endocyst of polypide, but that in all prob- 

 ability the entire endocyst is pierced to a greater or less 

 degree. by aquiferous pores. These, however, in common 

 with other observers who have sought for them, I have 

 failed to detect. 



RESPIRATION. 



Our knowledge of this function and of the organs by 

 which it is performed is also very limited. 



Beyond the very probable fact that the tentacles are 

 the principal respiratory organ, but little is definitely 

 known. The structure of the first membrane of the en- 

 docyst investing them, covered as it is with cilia, and the 

 homological identity of these parts with the well-known 

 respiratory fringes of the Brachiopoda and the respiratory 

 sack of the Ascidia show them to have similar functions. 

 The first membrane, however, also extends over the ex- 

 ternal surface of the entire colony, and, although devoid 

 of cilia, probably aids in aerating the circulating medium, 

 whenever it is uncovered by the ectocyst. The admission 

 of water to the interior is another means for keeping the 



