TfIR MORPHOLOGY OP THI'] CITEI LOSTOMATA. 291 



a g'uide over which the tewtnclo sheath works in the move- 

 ments of protrusion and retraction. I have not definitely 

 settled whether the calcareous film is a superficial calcifica- 

 tion or an internal cr^q^tocyst, but the existence of a delicate^ 

 free, vertical edge all round the " aperture " probably in- 

 dicates that there is living tissue adjaceiit to it (which I 

 believe that I have been able to make out), and that the film 

 is of the nature of a cryptocyst. 



The operculum is like that of Flustra, and there is a 

 single pair of parietal muscles, as in Dime to pi a. 



Bugula neritina,^ L. 'J^he aperture is elongated, and 

 occupies the greater part of the length of the zooecium. The 

 parietal muscles ai-e arranged in a series of Flustra-like 

 groups, which lie very close to the lateral walls of the 

 zooecium. 



(b) Cribrilinidfe. 



It has long been known that the characteristic frontal 

 wall of certain Cribrilinidaj develops as a series of marginal 

 spines, originating from the jjeriphery of the aperture and 

 converging towards one another until they meet, and so 

 form a calcareous roof in which the intervals between the 

 bars remain either as simple slits or as a series of pores. An 

 excellent account of this process is given, for instance, by 

 Hincks (1880, p. 190). 



It is easy to see, by an examination of dry specimens of 

 various Cribrilinidas (e. g. C. annulata) that the opei-culum 

 is not continuous with the calcareous frontal wall, but with 

 the underlying frontal membrane. This may be observed 

 either in immature zooecia or in those from which the wall 

 formed by the union of the bars has been broken away. 

 Goldstein (1880, p. 48) has made the interesting observation 

 that in C. monoceros(?) a Rotifer swam into the space 

 between the bars and the frontal membrane. Although this 

 is not expressly stated, it presumably entered at the distal 



' Busk (1852), p. U. 



