THK MORPHOLOGY OF THE CHEILOSTOMATA. 335 



It may perhaps iudicate that the cryptocyst may have 

 co-existed with a Cribrilina-like frontal shield, and it is not 

 indeed impossible that the plate i)?. in fig-. 54 may represent 

 the cryptocyst. 



Calvet (1900, pp. 163,436), who gives an excellent account 

 of the structure of many marine Ectoprocta, has already 

 expressed the opiuion that in Lepralioid forms the calcified 

 frontal wall is a cryptocyst. He states tliat while in most 

 species the cryptocyst and the more external '^ ectocyst " 

 (= epitheca) occur only on the frontal wall, in Retepora 

 the whole external surface is two-layered. This and the 

 analogous case of Urceolipora nana require further study. 



Calvet does not recognise the compensation-sac, although 

 it is indicated clearly in his pi. viii, fig. 9 (Catenaria 

 lafontii), and its development is partly described on p. 399 

 and figured in pi. xiii, fig. 20 (Lepralia foliacea). Tiie 

 account which he gives (p. 168) of the median pore of 

 Microporella does not agree with my results. I think 

 that the discrepancies are easily to be explained by4he fact 

 that in highly calcified species the compensation-sac is so 

 delicate that it may easily be overlooked in sections which have 

 been distorted by decalcification and embedding in paraflSn. 



Calvet, after quoting in extenso (p. 276) Jullien's very 

 accurate account of the mode of articulation of the operculum 

 and its relations to the compensation- sac, proceeds to give it 

 a complete denial (p. 278). He explains the existence of the 

 parietal muscles in species with a calcified frontal wall by 

 supposing that wall to have an amount of flexibility sufficient 

 to allow itself to be depressed by the parietal muscles. The 

 fact that these muscles are not inserted into the Cidcitied 

 wall is a sufficient answer to this sug-e'estion. 



Euthyris may be regarded as a highly modified Escharine 

 form, which has acquired a F lustra-like habit. The 

 irregularity of the bars of E. clathrata, and the fact 

 that they are entirely beneath the epitheca, suggest 

 that they do not represent the tubular frontal bars of 

 Cribrilina, but are due to deficiencies in the calcification of 



