i92 ZOOLOGY 



SECT. 



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. 139, 

 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 Medusa?. 



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. 137, 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 plamda, 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 would 

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

 single pair of mesenteries (Fig. 142; A, 1) grow from the body-wall to the gullet, 

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

 diameter of the stomodseum, 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 mesen- 

 teries (2) appear in the dorsal chamber, dividing it into a median and two lateral 

 compartments ; then a third pair (3) in the ventral chamber, producing a similar 

 division ; then a fourth pair (4) in the middle compartment of the dorsal 

 chamber ; then a fifth pair (B, 5) in the lateral compartments of the dorsal 

 chamber; and a sixth (6) in the lateral compartments of the ventral chamber. 

 Soon the longitudinal muscles are developed, and the fate of these primitive 

 pairs of mesenteries can be seen. The third and fourth pairs become the two 

 directive couples of the adult ; another couple of primary mesenteries is consti- 

 tuted, on each side of the vertical plane, by one of the mesenteries of the first 



