88 Rev. T. Hincks's Critical Notes 



in the zocecial orifice. Such appears to be Dr. Koschinsky's 

 opinion. Smitt, from the use which he makes of this cha- 

 racter in his classification, must have found reason for 

 believing- in its general constancy as well as in its signifi- 

 cance. Granting a certain amount of variability, it is only 

 what we might expect, for variability in greater or less 

 degree is met with in every element of organic structure, and 

 supplies the material, as we know, with which natural selec- 

 tion works in the evolution of new forms. It would be strange 

 if any form of structure were free from variation ; but in the 

 case before us, so far as my observations enable me to judge, 

 there is, as I have said, no special instability, but, on the 

 contrary, a remarkable constancy. 



In my ' History of the British Polyzoa ' I have carefully 

 noted the " range of variation " for a large proportion of the 

 species. An analysis of the observations recorded under this 

 heading will show that the zocecial orifice is one of the most 

 stable structural elements and that the amount of variation 

 which it actually undergoes is for the most part trifling both 

 in amount and significance. Of course this remark applies to 

 the adult primary orifice. 



Jn some species (belonging to various genera) there is a 

 difference which may truly be called remarkable between the 

 orifice of the ordinary and that of the ovicelligerous cells. 

 Before an ocecium has made its appearance in the colony our 

 attention is arrested by the presence of two dissimilar classes 

 of zooecia, in one of which (the less numerous) the orifice is 

 not only of abnormal size, but of equally abnormal form. 

 The latter will in time bear the ovicells and is modified with 

 a view to this function. We have good illustrations of this 

 peculiarity in Cribrilina clithridata, Waters, and Schizo- 

 porella longirostrata, Hincks. Of course this is not a case of 

 varietal difference ; the diversity of form is a specific charac- 

 ter and for a special purpose. It is quite possible, however, 

 that in the absence of the ocecia this twofold structure of the 

 cells might be misinterpreted, and might be classed as one of 

 the " remarkable differences " which go to prove the instability 

 of the characters of the orifice ; and I have therefore thought 

 it well to direct attention to it here *. 



* Smitt has noted the occurrence (in Escharella rostrigera) of larger 

 zooecia amongst the ordinary ones, exhibiting a great difference both in 

 the form and size of the orifice. Ocecia, as far as he could see, were 

 totally wanting, and he was unable to determine the physiological signifi- 

 cance of the difference. We can have no doubt, with our present know- 

 ledge, that the larger zocecia with the modified orifice were the zooids 

 deptined to bear the ocecia. 



