190 ZOOLOGY SECT. 



Hydra : the latter occur chiefly in the transverse muscular layer of 

 the tentacles and are endoderrnal, the longitudinal layer is formed 

 of distinct fibres of- ectodermal origin : the great muscles of the 

 mesenteries are of course endoderrnal. Although always derived 

 either from the ectoderm or endoderm, many of the muscle-fibres 

 of Tealia undergo a remarkable change of position by becoming 

 sunk in the mesoglcea, and thus appearing to belong to that 

 layer (Fig. 140 l.m.}. This fact is significant from the circum- 

 stance that, as we shall see, the muscles of all animals above 

 Ccelenterata are mesodermal structures. 



The nervous system is very simple. It consists of a layer of 

 delicate fibres lying between the epithelial and muscular layers of 

 the ectoderm. Among the fibres are found nerve-cells (Fig. 140, 

 nv.c), often of large size, and occurring chiefly in the disc and 

 tentacles. Thus, as in the polype-forms previously described, the 

 nervous system is in a generalised condition, and shows no con- 

 centration into a definite central nervous system such as occurs 

 in Medusae. 



Reproductive Organs. Sea-anemones are dioecious, the sexes 

 being lodged in distinct individuals. The gonads ovaries or testes 

 are developed in the substance of the mesenteries (Fig. 138, gon.), 

 a short distance from the edge, and, when mature, often form very 

 noticeable structures. The reproductive products are obviously, as 

 in the Scyphozoa, lodged in the endoderm. The sperms, when 

 ripe, are discharged into the stomach and escape by the mouth : 

 they are then carried, partly by their own movements, partly by 

 ciliary action, down the gullet of a female, where they find their 

 way to the ovaries and impregnate the eggs. 



The development of Sea-anemones resembles, in its main 

 features, that of Scyphozoa. The oosperm undergoes more or less 

 regular division, the details differing considerably in individual cases, 

 and becomes converted into a planula, an elongated ovoidal body 

 with an outer layer of ciliated ectoderm and an inner layer of large 

 endoderm cells, surrounding a closed enteric cavity, usually filled 

 with a mass of yolk, which serves as a store of nutriment. 



In this condition the embryo escapes from the parent, through 

 the mouth, swims about for a time, and then settles down, becom- 

 ing attached by its broader or anterior end. At the opposite or 

 narrow end a pit appears, the rudiment of the stomodseum ; this 

 deepens, and, its lower or blind end becoming perforated, effects a 

 communication with the enteron. 



The mesenteries are developed in regular order, but in a way which \vould 

 certainly not be suspected from their arrangement in the adult. First of all, a 

 single pair of mesenteries (Fig. 143, A, /) grow from the body- wall to t lie gullet, 

 being situated one on each side of the vertical plane, at right angles to the long 

 diameter of the stomodaeum, and near one end of that tube. The enteron thus 

 becomes divided into two chambers, a larger or dorsal and a smaller or ventral, 

 and the embryo acquires a distinct bilateral symmetry. Next a pair of rnesen- 



