OBSERVATIONS ON POLYZOA. 215 



seem to be expended entirely upon the evaginatory sys- 

 tem, and do not reach the lophophoric except in so far as 

 they increase its distance from the anterior pole by in- 

 creasing the evagination of the tube. 



The ccenoscial is also invariably the first region to 

 assume the full characteristics of the adult zooid, the 

 evaginatory next, and the lophophoric last. The bud, as 

 previously observed, is developed from the wall of the 

 cell, which last becomes well formed and distinct before 

 the alimentary canal loses the peculiar shortness and 

 breadth of the younger stages of growth, such as is figured 

 in PL 8, fig. 2, and not until long after both regions have 

 attained their full growth, do the tentacles on the ends of 

 the arms acquire the adult length. 



The three regions are equally well marked among the 

 Polyzoa of simpler organization than the Phylactolaemata, 

 with the exception of the genera Urnatella and Pedicel- 

 lina. The two latter have all three regions consolidated 

 in one, and there may be said to be but two systems in 

 their structure, the ccenrecial and lophophoric, the evag- 

 inatory having entirely disappeared, and the alimentary 

 canal and reproductive organs occupying the ccenoecial 

 region together. 



The exceeding simplicity of the alimentary canal, the 

 approximation of the mouth and anus, especially in 

 Urnatella, where, according to the unpublished plates of 

 Dr. Leidy (PL 15, fig. 5), they are so close together, 

 that they are separated only by their own membranes 

 which are continuous with each other, so that the intes- 

 tine and ossophagus in* the young seem to have but one 

 common aperture through the disk (PL 15, fig. 6) ; the 

 absence of the gastric ccecum of the Phylactolfemata, of 

 the pointed cesophagal valve, and of the evaginable tube 

 are all characteristics, that not only separate these two 

 genera from the Phylactolaemata, but show them to be the 

 very lowest of the Polyzoa. 



The lophophore is withdrawn within the cell, and the 

 invaginated fold forms in both genera a net between the 

 tentacles, resembling the calyx of the Phylactolaemata, to 

 which the lower part of the tentacles are soldered on its 



