ZOOLOGY. 





the flask be then closed with a plug of cotton-wool, 

 the liquid may remain for months quite clear. 



It is very difficult so to treat a vegetable or 

 animal infusion, and to prevent even one of these 

 infinitely minute and universally distributed par- 

 ticles getting into it during the process. The 

 consequence is, that in many flasks, notwithstand- 

 ing all precautions, bacteria do appear. One 

 negative in such an investigation is, however, 

 worth a thousand positive observations, and it 

 would seem that the number of negative results is 

 in proportion to the pains taken, and to the skill 

 of the experimenter. All present evidence goes 

 to shew that under no circumstances with which 

 we are acquainted, does any living being arise, 

 except as the progeny of a pre-existing living being 

 of the same kind. 



II. SUB-KINGDOM CCELENTERATA. 



The C&lenterata (Gr. koilos, hollow, enteron, an 

 intestine) have their parts arranged round a central 

 axis in a radiate manner, like the segments of an 

 orange, and they are capable of being split into 

 right and left similar halves. They are thus said 

 to have radial and bilateral symmetry. Further, 

 each segment of the body can be divided into 

 two symmetrical halves. They all have a distinct 

 digestive cavity, which always communicates with 

 the outer world by a mouth. In some, the diges- 

 tive cavity is identical with the body cavity, the 

 stomach being merely hollowed out in the gelatin- 

 ous body substance ; and in others it is distinct, 

 the wall of the stomach hanging as a free sac 

 within the body cavity ; but in this latter case 

 there are always free channels of communication 

 between the digestive sac and the general cavity 

 of the body. In the great majority, no circulatory 

 or nervous system is developed. 

 The body-wall is composed of two layers (ecto- 

 derm, outer skin, and endo-derm, 

 inner skin), which contain a number 

 of peculiar bodies termed ' thread- 

 cells.' These thread-cells consist of 

 an oval bag filled with fluid, within 

 which is coiled a long and extremely 

 slender filament provided with three 

 recurved darts or barbs. On the 

 application of the slightest force to 

 the tensely filled bag, this filament, 

 often several times longer than the 

 One of the larger ba ? is projected outwards, and trans- 

 thread - cells fixes and paralyses the prey, while 

 of the Hydra, t ^ e fl u j<i seems at the same time to 



with its three . . , . . 



recurved barbs, exercise a benumbing influence upon 

 it. Distinct reproductive organs 

 are present in all. The Ccelenterata are divided 

 into two classes. 



CLASS I. HYDROZOA. 



The digestive cavity is not separated from the 

 body cavity, but is identical with it, and the repro- 

 ductive organs are on the external surface of the 

 body. Hydra is the type. It is usually seen 

 attached to some aquatic plant or twig in our 

 ponds and ditches. It is cylindrical in form 

 and gelatinous in consistence, and usually about 

 half an inch in length. It has at one end an 



Hydra fusca: 

 .., 



ith a young bud at 6, and 

 more advanced bud at c. 



expanded disc, by which it attaches itself to any 



object; at the other is the 



mouth, which is surrounded 



by a Circle of arms or ten- 



tacles, which move about in 



all directions in the water. 



The body is composed of 



the two layers, armed with 



thread-cells, which are spe- 



cially abundant in the ecto- 



derm and in the tentacles. 



Small larvas and worms 



are its favourite food. To 



entrap these, it spreads out 



its tentacles, moving them 



gently in the water, to in- 



crease their chances. The 



prey is paralysed by means 



of the thread-cells, and is 



pushed through the mouth 



into the internal cavity, 



where it undergoes diges- 



tion, the indigestible por- 



' , . / 11 11 j 



tions being finally expelled 



through the mouth. It is 



not permanently fixed, but can detach itself, and 



move about in the water. 



Trembley of Geneva shewed that Hydra is 

 exceedingly tenacious of life, and endowed with 

 extraordinary reparative powers. It may be turned 

 inside out and still live, or be cut into pieces, each 

 of which may become a perfect hydra. It increases 

 towards the end of the year by ova ; and during 

 the summer months, by buds or gemma, which 

 sprout from the sides of the parent. At first, 

 the stomachs of the bud and parent are con- 

 tinuous ; but they become separated by a constric- 

 tion, tentacles are formed at the free end, and it 

 then separates as a perfect hydra. 



ORDER i. Hydridce includes only the genus 

 Hydra. Imagine a hydra going on budding, and 

 the buds remaining attached to the parent a 

 tree-like arrangement would thus be produced. 



ORDERS 2 and 3. This is almost constantly the 

 case in the second and third orders, Corynidce 

 and Sertularidce (Sea-firs), and the animal is thus 

 composite. It is usually permanently fixed, and 

 is inclosed in a firm but flexible tube, which ex- 

 tends only up to the base of the buds or polypites 

 in the Corynidcej but in the Sertularidce forms a 

 little cup for each polypite, into which it can 

 retract itself when alarmed. They are often found 

 on the sea-shore, attached to old shells. The 

 reproductive process in the Corynidte and Sertu- 

 laridce is certainly very peculiar. At certain 

 seasons, some of the buds become modified in 

 form and structure, so that they assume an 

 appearance not unlike a Medusa. To these buds 

 the term zooid is applied. After a time, these 

 zooids or Medusa-like buds become detached 

 from the parent stem, and swim about for some 

 period, when there are developed in them the 

 organs of reproduction. So closely do these 

 zooids resemble the true Medusidce, that they 

 were for a long time confounded with them. The 

 ova and spermatozoa of these zooids now unite, 

 and give rise to bodies having the appearance of 

 a flattened disc covered with cilia, by means of 

 which they swim freely about. This disc-like 

 body becomes attached, part of it forming the root 

 by which it is attached, while from its centre there 



