POLYPIFERA. 



43 



The growth of the horny polypary exactly 

 keeps pace with the development of the soft 

 substance, and even goes beyond it. Bdow 

 the tentacula the body soon becomes con- 

 stricted, marking the boundary between it 

 and the stem; and soon the polype, becoming 

 too large to be contained in its sheath, issues 

 forth, and expanding its tentacula becomes 

 perfectly unfolded. The oviferous pedicles, 

 hereafter to be described, are developed sub- 

 sequently. 



Fig. 48. 



Tubularia coronata, magnified. 



a, stalk ; b, walls of the polypary ; c, substance 

 common to all the individuals, whereby they are 

 brought into mutual organic communication ; d, 

 limit between the individual and the community ; 

 g, the long tentacles ; h, the short tentacles ; k, col- 

 lar formed by the tentacles ; o, ova ; n, a bud ; p, a 

 bud further developed; q, a bud still further ad- 

 vanced, showing indications of the two rows of 

 tentacles (g, h). (After Van Beneden.} 



Second mode of propagation, by free gemmce. 

 The free gemmae are produced upon dis- 

 tinct pedicles, which in the genus Tubularia 

 are developed within the lower circle of ten- 



tacula. They resemble numerous appendages 

 disposed in a circle and forming a crown 

 around the body of the polype. (Fig. 48, o.) 

 These pedicles grow in the same manner as the 

 buds and the tentacula described above, that 

 is to say, a hollow tubercle first makes its 

 appearance, which seems to be merely an 

 extension of the external covering of the 

 polype. Each tubercle slowly expands, and 

 soon divides into one or more branches, 

 which are all hollow, and the same fluid 

 which circulates in the general substance of 

 the polype may be observed to pass 

 into their interior. 



At the free extremity of each of 

 the pedicles thus formed a distinct 

 cell is soon perceptible, situated im- 

 mediately beneath the surface, which 

 cell is the rudiment of a new in- 

 dividual. (Fig. 49, 1,0.) No nucleus 

 has been remarked in its interior. 

 This primitive cell, which might also 

 be regarded as an egg or as an 

 ovule, sometimes becomes organised 

 internally, in which case the repro- 

 ductive process assumes the third or 

 the fourth form, subsequently to be 

 noticed, or else it serves for the 

 point of departure, or it might 

 almost be said the mould for the 

 formation of a free gemma, which 

 becomes organised around it at the 

 expense of the pedicle itself. It is 

 in effect a part of the reproductive 

 appendage that will subsequently 

 become detached ; but at this period 

 of its development it is impossible 

 to determine after which of the four 

 modes of reproduction the embryo 

 will be formed. The vesicle () 

 now increases rapidly in size, and 

 beneath it another membrane is 

 soon perceptible, which by its inner 

 surface is in contact with the cir* 

 culating fluid. This membrane is 

 the origin of the new individual, 

 or, in other words, a blastoderm, 

 formed by the internal skin, and not 

 by the vitellus. Soon there is seen, 

 projecting from its centre,a little cone 

 (fig, 49. 3, 4), which, compressing 

 the vesicle (), forms a depression 

 upon its inner surface, and the 

 vesicle now begins to assume the 

 appearance of a serous membrane, 

 yielding to the pressure of the organs over 

 which it spreads, and ultimately covers, much 

 in the same way as the pleura covers the lungs. 

 The tubercle (b) will afterwards form the walls 

 of the digestive cavity, and may be seen to have 

 the circulating fluid derived from the body of 

 the polype moving in its substance. Around 

 the base of the cone (b) may now be seen four 

 other tubercles (r, 4, et seq.), which become de- 

 veloped like the preceding; but, instead of 

 compressing the vesicle (a), they surround it, 

 and ultimately completely enclose it. They 

 carry the skin with them, so as to have the 

 appearance of a transparent vase, having four 



