Prof. W. King on Pleurodictyura problematicum. 139 



through the substance of the cell-walls : in no case can I perceive 

 any appearance as if it had passed through the cells themselves ; 

 nor have I ever seen anything to show that it cut, as it were, 

 any of the foramina* : on the contrary, the cells have been built 

 around, or adjacent to, the appendage ; and the foramina have 

 been carried over or under it. 



These facts I regard as completely proving that the original 

 of the vermiform appendage was an ab initio structure — a true 

 integral organ of Pleurodictyum. 



From the absence of all evidence to the contrary, it cannot be 

 concluded that the appendage was enclosed in a sheath, like that 

 of some shell-bearing borers f : it must be the cast of a fleshy 

 tube included in, and protected only by, the substance of the 

 cell-walls. 



The reader will now be prepared for the statement, that I 

 consider the vermiform appendage as the cast of a tubular 

 chamber which enclosed the intestinal canal of Pleurodictyum. 



J3ut an intestinal canal necessitates the existence of a mouth 

 and an anus. No coral, however, possesses more than one orifice 

 to its digestive cavity. Pleurodictyum therefore cannot belong 

 to any division of the class Corallaria : it must be assumed as 

 belonging to a higher type in the scale of animal organization. 



Considering certain primary groups of the Invertebrata pos- 

 sessing an oral and an anal orifice — for example, Tunicata, Mol- 

 lusca and Bryozoaria — if all their existing and extinct represen- 

 tatives be examined, we shall fail in observing any to be strictly 

 available for Pleurodictyum. This being the case, our only alter- 

 native is to have recourse to its collateral affinities, which, as 

 already seen, lean to Zoantharia. Reflecting on this order, it 

 may be asked — of the classes named, having two orifices, which 

 one has the closest relation to it ? Obviously Bryozoaria J. But 

 in no cell-bearing forms of this class, such as Lepralia, Eschara, 

 &c., is there one to be found consisting of large cells with a ver- 

 miform appendage : all of them have their solid parts consisting 



* The vermiform appendage having increased in size as the fossil became 

 enlarged, thereby encroaching on the adjacent cells, it must not be sup- 

 posed that cases exhibiting the foramina broken through do not occur. It 

 may be observed, however, that although I have seen thread-like casts of the 

 foramina almost, if not completely, touching the appendage, I have never 

 perceived any appearance as if they had been cut by a boring animal. 



t The cases of close proximity alluded to in the above note completely 

 dispel the idea that the appendage originally possessed a shelly sheath, as 

 some have conjectured. 



X It may be readily imagined that the class Tunicata occurred to me as 

 a group to which Pleurodictyum might belong ; but as its collateral aflfi- 

 nities lean rather to Mollusca than to the Zoanthic corals, I felt myself 

 compelled to abandon the idea. 



