538 



Marvels of the Universe 



Photo iy] 



'est, L.D.S. 



SEA-FIR. 



This species is known as tKe Sea-Hair Coralline. 

 Its small cells and nursery capsules will be seen to 

 differ in shape from those of the Sea-Oali Coralline. 

 (Enlarged about fifteen times.) 



the former is broader in proportion to its length. The 

 Hydra, it will be remembered, is a little bag, closed at 

 the lower end and with only the mouth opening at the 

 upper extremity in the centre of the fringe of tentacles. 

 Its walls consist of three layers of flesh, an outer 

 and an inner coat with a jelly-like layer in between. 

 From certain cells of its outer coat it is able to eject 

 long stinging threads which kill or paralyze its prey. 

 Now in a general way the " Hydroids," or Hydra- 

 like polyps that live in the cells of the Sea-Firs 

 agree in structure with the Freshwater Hydra, ex- 

 cept that some of them do not possess the stinging 

 threads. 



The beginning of the Sea-Fir colony is an egg 

 which, on hatching, produces a swimming larva, 

 but this soon settles down and develops into a 

 creeping, root-like tube attached to shell or seaweed 

 or some other firm base, with a single Hydroid in its 

 little cell. This produces a bud, which develops 

 into another Hydroid, say, on its left side. The 

 second Hydroid produces a bud on its right side ; 

 and so with this continual alternate budding we 

 soon have a stem closely beset with cells and 

 Hydroids on each side. At intervals a Hydroid buds on both sides, with the result that a branch 

 stem starts off. and so the tree-like colony arises. If we look at the photograph on page 535, which 



shows a few Hydroids of a different family, as they 

 appear under the microscope, we shall be struck 

 with the general resemblance between these minute 

 animals and the Sea Anemones, already described 

 in a separate article. Hydra, H^^droids, Sea Ane- 

 mones and Coral Polyps are constructed on much 

 the same lines, and are regarded by naturalists as 

 all members of one great class, though each of the 

 four animals named represents a distinct family in 

 that class. 



One of the species photographed (page 536) is 

 known as the Sea-Oak Coralline, in which the 

 regular arrangement of the horny cells on the 

 stems and branches produces a resemblance to the 

 backbone of a small fish, such as a sardine. 

 The photograph represents only a small portion 

 magnified, for the whole branching colony is only 

 about an inch in height. Each cell contains, or has 

 contained, a little polyp, which can, in the case 

 of any alarm, withdraw itself completely into its 

 cell. Some species have a sort of lid which closes 

 automatically when the poh'p retires. In winter 

 all the cells may appear to be emptj', but an 

 element of life remains at the bottom, and in spring 



Photo 6.u] 



[ir. West, L.D.a. 

 SICKLE CORALLINE 



The stem and branches are spirally twisted, and 

 the polyp-cells all stand on the upper side of the 

 branches. The four extra large cells shown near 

 the stem are the nursery cells in which the young 

 are produced. 



