CTENOPHORES. 



475 



ever, such as the sea-anemones and the jelly-fish, have no hard skeleton at all, but 

 are amongst the most delicate and beautiful objects in the realm of living nature. 



In spite of the variety of forms to be found, the Coelenterata are almost as 

 incapable of higher development as the Echinoderms. Like the latter, they have 

 failed to make any way in fresh water, not to speak of the land. A few free- 

 swimming jelly-fish, a minute attached polyp, and some degenerate sponges are, 

 indeed, found in fresh water, but these can hardly be looked upon as successes. 

 While, at present, it is not easy to connect the Coelenterata with any other group, 

 inasmuch as they appear to stand without any near relatives among the higher 

 animals, they have a special interest, since they are considered to represent a stage 

 in the development of animal life through which all the higher forms have passed. 

 Some simple form of Coelenterate may have given rise to all the higher animal 

 forms, the modern Coelenterates — the sea-anemones, corals, etc — being those 

 descendants of the primitive simple form which have retained the original type of 

 organisation almost unchanged. 



The Ctenophores, — Group Ctenophora. 



Although all are agreed that the so-called Ctenophores are members of this 

 subkingdom, their exact position is not clear. The Ctenophores are glassy, 

 transparent creatures, either shaped like apples, melons, or Phrygian caps, or else 

 forming bands, often a yard in length, and thickened at the middle. Several 

 types are shown in the coloured Plate. The marvellous transparency of all 

 but one (Beroe) is specially remarkable. They inhabit the open sea, or are driven 

 by currents and winds near the coast and into harbours. Their position in 

 the water is usually more or less vertical, the mouth being turned downwards. 

 The organs from which this group takes its name are the ribs, which either run 

 from pole to pole, or else only for certain distances along the meridians, which are 

 often symmetrically arranged. These ribs consist of rows of short transverse 

 combs, each being formed of a row of cilia. The cilia forming a comb are connected 

 together at their bases, but are also capable of independent movement. As they 

 wave to and fro, they constitute what is called a swimming or rowing plate. The 

 activity of these rows of plates depends upon the will of the animal, which can 

 move either the plates of a single rib, or all the ribs together ; this latter movement 

 resulting in slow locomotion in the direction of the apical pole, i.e. the pole turned 

 away from the mouth. The body is capable of various swift, light, and graceful 

 movements, for in addition to the rowing plates there are other structures, such as 

 the oral umbrella and the capturing filaments or tentacles, with their hair-like 

 branches. These tentacles, which are attached like arms at the sides, are capable 

 of erection, or of withdrawal into pockets. There is great variety in the develop- 

 ment of these accessory organs of locomotion. For instance, the Cydippidoi 

 have only arms, which, with their branches, serve for capturing food as well as 

 for steering. In other orders, vertical, oar-like, dermal folds stand out from the 

 body, by means of which the movements become more rapid and energetic. Some 

 species of Eucharis, by suddenly shutting up the oral umbrella, can jerk themselves 

 forward ; and when successive jerks of this sort cause the body to move with 



