DEVELOPMENT. 73 



The upper surface of the disc has been all along covered with a thin layer of chitiuc, whose 

 periphery is continuous with the chitinons walls of the cup, but which does not interfere with the 

 growth of the young hydranth ; for as the latter continues to extend itself, the layer of chitine on 

 the upper surface of the disc is carried onwards before it, without becoming thereby detached 

 fi'om the side of the cup — a fact which we can scarcely explain otherwise than by supposing con- 

 siderable extensibility in the recently deposited chitine of the cup. At hist the hydrotheca has 

 attained its complete size and shape, and now the young hydranth becomes more or less retracted 

 within it, the terminal plug-like disc withdrawing itself from the layer of chitine which it had 

 excreted on its upper surface, and which is now left behind as a roof closing over the mouth of 

 the cup. 



Tiie whole circumference of the retracted disc now begins to develop a circle of minute 

 tubercles (e), which gradually elongate themselves into short thick tentacles, while the central part 

 becomes elevated into a blunt conical proboscis (hypostome), and the cylindrical tubular column 

 which occupies the axis of the hydrotheca has become dilated into a more oval-shaped body, with 

 a wide internal cavity — the stomach of the developing hydranth. 



The young hydranth, still included within a completely closed cup, presents greater and 

 greater contractility, now withdrawing itself towards the bottom, and now extending itself through 

 the entire height of the surrounding cup. The tentacles in the mean time have become longer, 

 the extremity of the terminal cone has become perforated by a mouth, and at last the hydranth 

 pushes off the chitinous roof of its hydrotheca, and emerges into free contact with the surrounding 

 water. 



h. Bevehpment of the BlastostyJc. 



In the gymnoblastlc genera the development of the blastostyle is essentially similar to that of 

 the first stages of the hydranth. Instead, however, of proceeding to the development of prehensile 

 tentacles, an arrest takes place, sporosacs bud from its sides, and the nutrition of the colony, to 

 which the hydranth is destined, becomes replaced by the duty of supporting the sexual buds. In 

 the calyptoblastic genera the development of the blastostyle is accompanied by some additional 

 features which render necessary a more detailed description. 



Laomedea Jlewuosa wiU afford here too a very convenient subject for tracing the process of 

 development. The blastostyles of this hydroid arise close to the axillae of the branches, and 

 present the form of a long cylindrical column, expanded at its summit into a disc, occupying the 

 axis of a spacious gonangium, and carrying along its whole length adelocodonic gonophores, which 

 increase in maturity as they approach the summit of the column. The whole is elevated on a 

 short annulated peduncle. 



The blastostyle here originates in a bud precisely in the same way as a hydranth ; and up to 

 the stage to which we have already followed the development of the hydranth and hydrotheca, 

 when these parts present the condition of a conical enlargement of the extremity of the branch, 

 there cannot be found any difference between the hydranth-bud and the blastostyle-bud. It would 

 seem, however, that at this stage the soft parts, instead of absolutely withdrawing themselves 

 from contact with the external chitinous capsule, present in their ectodermal layer a number of 



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