12 Dr Harmer on the Structure and Classification 



Waters describes the compensation-sac, without understanding 

 its mode of action, in Galwellia sinclairii, C. bicornis, Urceolipora 

 dentata, Onchoporella bombycina (?), and Ichthyaria ocidata(t); 

 his account 1 appearing in the same year as Jullien's and quite 

 independently of it. 



It is perhaps owing to Jullien's remarkable views on the 

 subject of Zoological nomenclature that his results have been 

 either ignored or discredited by later observers. The existence of 

 the compensation-sac is denied by Pergens 2 for species of Schizo- 

 porella in which it is certainly present ; and Levinsen 3 expressly 

 controverts the statement that the so-called " median pore" of 

 Microporella malusii is open to the exterior. 



I have been led to make observations on this subject as the 

 result of the examination of what I believe to be a new species of 

 Euthyris 4 , kindly sent to me from Port Jackson by Mr T. White- 

 legge. The study of this species completely confirms the accuracy 

 of Jullien's statements. I am fully in accord with him in 

 attaching importance to the characters of the " front-wall," or 

 " paroi frontale," as a guide to the classification of the Cheilosto- 

 mata. His statement that the compensation-sac occurs " chez 

 toutes les especes de Cheilostomiens monodermies 5 " is a generali- 

 sation which does not appear to have been founded on a large 

 number of cases ; but it is nevertheless in all probability sub- 

 stantially true. 



My own results may be summarised as follows : — 



(i) The central group of the Cheilostomata is constituted by 

 such families as the Membraniporidae, Flustridae, Farciminariidae, 

 etc. In these the front wall (= opercular wall) remains entirely or 

 to a large extent membranous, the membranous part being known 

 as the " aperture." The operculum is a moveable part of this 

 membrane, its base-line not being usually strengthened by a basal 

 sclerite, the presence of which would introduce rigidity where 

 flexibility is most wanted. A series of parietal muscles originates 

 from the lateral wall of the zooecium, on each side, and is inserted 

 into the membranous aperture. The contraction of these muscles 

 exerts a pressure on the fluid of the body-cavity, and thereby 

 causes the protrusion of the polypide ; — a mechanism described 

 by Nitsche in 1871 6 . 



(ii) A second group of Cheilostomata is constituted by the 



1 Challenger Reports, "Polyzoa Suppl." Part 79, 1888, pp. 17, 18, 3, 10. 

 3 " Untersuchungen an Seebryozoen," Zool. Anz., xn. 1889, p. 507. 



3 "Polyzoa, Hauchs Togter," Copenhagen, 1891, p. 285. 



4 I hope to publish an account of this species in a paper dealing more fully with 

 the compensation-sac. 



5 Mem. Soc. Zool. France, i. 1888, p. 275. The " monodermies " correspond for 

 the most part with the Lepralioid or Escharine forms of other authors. 



6 Zeitschr. iviss. Zool., xxi. p. 426. 



