576 



THE HYDRQIDA. 



is worthy of notice, that the Gorganiae are never bushy, and for the most part have their 

 branches in the same plane. 



In this illustration we have an example of a very interesting 

 and extremely beautiful species. 



The SEA-PEN is so named because its whole form bears the 

 most remarkable resemblance to a quill-feather, consisting of a cen- 

 tral shaft, from which a double row of "pinnae" is developed at 

 right angles, bearing the polype on their upper margin. As may 

 be seen by the illustration, the whole form of this curious being is 

 remarkably graceful, and it really seems as if it had been modelled 

 upon a quill-feather plucked from the wing of some bird. 



The Sea-pen is never attached to solid substances, but remains 

 quite free in the ocean. It does not, however, swim, but is a help- 

 less sort of being, and only kept in its proper position by the base 

 being thrust into the mud or sand at the bottom of the sea. Some 

 species of Sea-Pinnje are phosphorescent, and present a magnificent 

 sight in the darkness. It was once thought that the creature was 

 able to swim by means of the webs, or pinnae, which flapped like 

 the fins of a fish, but it is now ascertained that no such power 

 resides in these organs. The stem is of a rather soft consistency, 

 strengthened by a bony centre, which reaches nearly to its tip. 



An object of somewhat similar form, but considerably elon- 

 gated, and with the pinnae proportionately shorter, is called SEA- 

 BUSH ( Virgularia mirabttis), an animal belonging to a genus 



that can easily be distinguished from the preceding by several peculiarities. The pinnae 

 are short, deeply scooped above, and, with their bases, partly surround the central stem. 

 The polypes are set only upon the edges of the pinnae. There is an allied species belonging to 

 the same family, called by the name of Pavonaria. In this remarkable genus, the general 

 shape of the lengthened mass is four-sided, and the polypes are arranged in a somewhat 

 spiral form on the stem, but only one side of its latter half. In temperate seas, the Sea-rushes 

 do not grow to any great length ; but under a tropical sun they reach great dimensions, some 

 of them measuring more than a yard in length. 



A very curious inhabitant of some seas, which is in the habit of encrusting all kinds of 

 marine bodies, such as shells, stones, and stems of the large algae, is popularly known under 

 the name SEA-FINGER (Alcyonium. digitatum). Its general mass runs into lobes, and is of a 

 soft, spongy consistency, pierced with little holes, from which the polypes make their appear- 

 ance when in health. When closely examined, the little holes or pores are seen to be formed 

 of eight rays, in a kind of star-like pattern, and corresponding to the tentacles of the polypes 

 which inhabit them. These little cells are placed at the ends of canals, which permeate the 

 whole mass, and serve to unite into one common body the vast number of polypes which are 

 thus aggregated together. When examined by the microscope, the substance of the polyp- 

 idon is found to be filled with tiny particles of chalky matter, which serve to give consistency 

 to the fabric, and add to its elasticity. 



SEA PW.Permatiita grteca. I.ODB- 

 qnarter natural size, b, Enlarged 

 cell.) 



HYDROID A. 



WE now arrive at the order Hydroida, which are known by the internal cavity being 

 simple, and the creature increasing by buds thrown out from the sides. The Tubulariadse are 

 the first family of these creatures. In the Tubulariadae the buds grow from the base of the 

 tentacles, and break off their attachments as soon as they have attained maturity. The buds, 



