278 NATURAL HISTORY. 



are rather exceptional, for the greater part of the free-swimming Medusae, or Jelly-fish, are the 

 highest developments of individuals which began life in a different shape, and had different habits. 

 The fixed and polype-looking kinds, which have a branched stem, and on it one kind of zooid for 

 nutrition and another for reproduction, develop in certain receptacles of this last, either as larvae, 

 which escape as ciliated elongate or globular bodies that settle down and become like their parent, 

 or else as piano-blasts wandering buds Jelly-fish or Medusae which, when they escape, grow and 

 develop sexual elements, and their eggs hatch into the shape of the young individuals of the fixed 

 colony. The generation is then said to be alternate. It is probable, however, that the rudiments 

 of the contents of the generative sacs are developed within the central canal of the body, and pass 

 thence into special organs, and grow into shape. Budding also occurs, and similar forms are 

 reproduced by it. Usually there is great transparency of the tissues, and cilia exist on some kinds, 

 and all have sting- or nettle-cells, or nematocysts in their derm. These are cells with a spiny thread 

 coiled up in them, which escapes on pressure or irritation. The touch of the fine thread, with 

 or without the contents of the cell-sac, produces a paralysing influence on minute Crustacea and 

 animalcules, which form the bulk of their food. 



The contrast in the dimensions of the Hydrozoa is remarkable ; some of the Jelly-fish are several 

 feet in diameter, and others are like little balls, and the branching or fixed kinds may be microscopic 

 or some inches in length ; the first are muscular in some parts, and the last are more or less chitinous 

 in their investment. Special senses are represented in the free forms by eye -spots and minute 

 particles of mineral matter or lithocysts, and in most the tentacles which surround the region of 

 the mouth or the margin of the disc of the Medusae are retractile, and are weapons of offence or of 

 capture. The nervous system is very rudimentary, being more or less in connection with the muscular 

 fibres, in some being made up of nervo-muscular tissues, contractile and sensitive, in the meso-derm, 

 or middle-skin. Haeckel has described a circular band of nerve, on the inner side of the circular canal 

 of the ball-shaped Medusae, and states that it gives off shoots to the lithocysts, radial canals, cavity, 

 and mouth. But the evidence is not very satisfactory. There is no circulatory system, properly 

 speaking, and no special blood ; and the juices of the body are aerated through the delicate tissues. 

 All are aquatic. The Hydrozoa are divided into five orders the Ctenophora, Discophora, Siphono- 

 phora, Hydroida, and Hydrocorallina. 



ORDER CTENOPHORA. 



These are free-swimming Hydrozoa, usually globular or cylindrical in shape, and rarely ribbon- 

 shaped, and they are more or less lobed. They have rows of flappers placed like lines of longitude on 

 their body, and sometimes two tactile filaments, which can be retracted. The 

 stomach is more or less tubular, and is associated with a series of canals. Never 

 budding, they do not produce colonies or compound organisms, and they are 

 characterised by the great development of the middle tissue, or meso-derm. A 

 nervous ganglion, at the side remote from the mouth, with eight radiating cords 

 to the paddles, appears to have been made out satisfactorily. 



The Ctenophowe,* not having a disc, and not resembling the Medusae, or 

 Jelly-fish, in their shape, have a totally different method of moving in the water. 

 Whilst the great Jelly-fish contract and expand their bodies in regular succession, 

 moving in a very stately manner, the Ctenophorse dart here and there, rapidly ascend, 

 descend, and move slowly at will ; so that at night, when the great Medusae are 

 phosphorescent, and look like pale, slowly-moving spheres under water, the little 

 Ctenophorae flash here and there with a bright light, and are soon out of sight. 

 They move by the rapid flapping of countless little paddle-like processes arranged 

 BEROE PILEUS. in vertical rows along the sui'face of the body, like the teeth of a comb. The 

 rows may all be in full vigour of movement, or one only may act; and, indeed, 

 separate paddles appear to move independently and at will. The little creatures thus rise and 

 move obliquely, or fall and progress, according to the quantity and the position of the skin 

 machinery which may be used. They can stop and float in mid- water, and again dart off; and A. Agassiz 



* Comb-bearers. 



