92 



further confirmation of these views, it may be briefly 

 noticed, that if the cells be placed in acetic or dilute 

 nitrous acid, all the carbonate of lime is removed, and the 

 undoubted organic portion of the cells looks but slightly 

 altered from what the cells were before the obliteration 

 took place. If the experiment be reversed and a speci- 

 men be boiled in caustic potash, the organic matter is 

 removed and the spongy calcareous case remains perforated 

 or porous from the removal of the matter which for- 

 merly pervaded the whole. Thus then it seems clear that 

 the polypidoms are organic* 



The mode of reproduction in this order varies a great deal, 

 and in many genera it is entirely unknown. In the genus 

 Crista, which we have noticed as resembling Sertnlaria, it 

 is effected by the periodic formation of ovarian vesicles. As 

 the formation of these cells is very similar to that of those 

 described among the Hydroidce, little need now be said 

 on the subject. In them the ova or gemmules are formed from 

 the vital pulp, which at first occupies the whole of the cavity, 

 but as developement advances, it is withdrawn towards the 

 centre. It is of the same consistency as the pulp which 

 traverses the centre of the polypidom, and in fact is a 

 continuation of it. It very soon becomes developed into 

 globular gemmules, which are clothed with numerous vibra- 

 tory cilia, that are in constant action. By these, after the 

 gemmules have escaped from the vesicle, they are whirled 

 about in a rotatory manner through the surrounding fluid, 

 like worlds in minature. Like the gemmules of th e Hydroidce 

 they at last become fixed, and like them spring up into deli- 

 cate and beautiful arboresence ; but they are calcareous 

 instead of horny. After having performed their functions, 

 these organs drop off and disappear, or are thrown oft' like 

 the leaves of trees in autumn. In the Hydroidce it has been 

 noticed, that these temporary organs are abundantly pro- 

 duced, and may, in one or other species be noticed at all 

 seasons of the year; here, on the contrary, they appear to 

 be of rare occurrence, for after long and attentive searches, 

 two specimens only have been procured. This paucity of 

 vesicles, however, may arise from the solid, calcareous, and 

 friable nature of the structure ; for as they stand on very 

 slender foot-stalks, they nm^t be incapable of resisting the 

 violence of the sea. This however can hardly be the 

 only reason, since many hundreds of specimens of each 

 species, have been examined at different seasons, in summer 

 and after long continued calm weather, with but little 



* This subject is further elucidated by M. Milne Edwards, Ann. des 

 Sciences Kat., vol. 1, p. 25. Johnston's Brit. Zooph., p. 327. 



