50 



THE POLYPI. 



S^^ 42, 43. 



CHAPTER VIII, 



ORGANS OF SECKETION. 



§ 42. 



Nothing like urinary organs have yet been found in Polyps. Perhaps 

 the borders of the mantles of the cellular Polyps should be regarded as 

 organs of special secretion, since by them the increase and production of 

 these cells take place.''' 



CHAPTER IX. 



ORGANS or GENERATION. 



§ 



43. 



Polyps reproduce by gemmation, fissuration, and by eggs. 



1. Ffs«^ra^fo?i is comparatively rare ; it takes place nearly always lon- 

 gitudinally, and the division may or may not be complete/'* 



2. Gemmation is their most common mode of reproduction. The new 

 individuals may be completely detached, or may remain connected with the 

 parent corallum. 



a: In gemmation, complete separation of the young individual is, on 

 the whole, rare. It is best known in Hydra, with which the buds 

 always appear upon a certain part of the body, — that is, at its union with 

 the foot.'-* A bud of this kind consists always of a simple fold of the wall 

 of the stomach and the skin, so that the stomach of the young individual 

 is in direct communication with that of the parent, and the chyme can pass 

 freely from one to the other. When the foot of this new being has acquired 

 a proper development, it is completely detached at its inferior extrem- 



b : Gemmation without separation of the new beings is quite common 

 with Polyps, and occurs with very various modifications. The buds are 

 formed sometimes upon the sides, sometimes upon the base of the body. 

 In the first case, the coralla have a dendroid aspect ; in the second, they 

 are more lamelliforra, spherical or lapidescent. These variations are not 

 limited to certain genera or species, being often due to external influences, 



1 The calcareous tubes of Tubipora, and the 

 corneous ones of the Sc-rtularhia and otlier IJi-yozoa, 

 are, without douht, secreted by the border of the 

 mantle, as is true of the shells of mollusks. 



1 Acconlin;; to Roesel (Ins'-ktenbelu^t. III. p. 

 504, 525. Taf. LXXXIII. tii:. 3\ fissuration takes 

 place transvcrs:!ly with Hydra. Lnn^itudinal 

 fissuration is jirincipally observ-J with the Mailre- 

 porina. When it is complete the cells of the coral- 

 lum are definitely limited, as in Astraea, Favia, 



and Carijophyllia ; but, when incomplete, the cells 

 are brancheil, lobulated, and of irregular contour, 

 as in Agaricia, Maeandrina,aTid Monticularia, 

 kc. 



2. RoeselQoc. cit. m.Taf. LXXXV.fig. 2, 3, 5, 

 Taf. LXXXVI. and LXXXVIII. fig. g. h. and Taf. 

 L.XXXIX. fig. 4). Tlie exceptions to this rule, 

 which are Bometimes observed, are probably due 

 to lesions of au accidental nature. 



