214 



ZOOLOGY 



SECT. 



v.rrt 



TLLL 



of the combs. Thus the sensory pit acts as a central nervous 

 system, and the ciliated grooves as nerves. A sub-epithelial plexus 

 of nerve-fibres with nerve-cells extends all over the surface of the 

 body, and nerve-elements are also traceable in the mesoglcea. 



Reproductive Organs. The animal is hermaphrodite, the 

 organs of both sexes being found in the same individual. The 

 gonads are developed in the meridional canals (Fig. 160, B), each of 

 which has an ovary (ovy.) extending along the whole length of one 

 side, a spermary (spy.) along the whole length of the opposite side. 

 The organs are so arranged that in adjacent canals those of the 

 same sex face one another. It will be seen that the reproductive 

 products have, as in Scyphozoa and Actinozoa, the position of 

 endoderm-cells : whether they are developed, in the first instance, 

 from that layer is uncertain. When ripe, the ova and sperms are 

 discharged into the canals, make their way to the infundibulum, 

 thence to the stomodaeum, and finally escape by the mouth. Im- 

 pregnation takes place in the 

 water. 



Development. The pro- 

 cess of development has been' 

 traced in several genera closely 

 allied to Hormiphora, so that 

 there is every reason to believe 

 that, in all essential particulars, 

 the following description will 

 apply to that genus. 



The egg (Fig. 164) consists 

 of an outer layer of protoplasm 

 (plsm.) containing the nucleus 

 (nu.), and of an internal mass 

 of a frothy or vacuolated nature (yk.) : the vacuoles contain a homo- 

 geneous substance which serves as a store of nutriment to the growing 

 embryo, and apparently corresponds with the yolk which we shall 

 find to occur in a large proportion of animal eggs. Enclosing the 

 egg is a thin vitelline membrane (v. m.), separated from the proto- 

 plasm by a considerable space, filled with a clear jelly. 



After impregnation the oosperm segments, but the details of 

 the process are very different from those we are familiar with in 

 the other Ccelenterata. The protoplasmic layer accumulates on 

 the side which will become dorsal, and the oosperm divides along 

 a vertical plane, forming two cells each with a sort of protoplasmic 

 cap (Fig. 165, A, plsm.). A second division takes place at right 

 angles to the first, producing a four-celled stage (B), and each of 

 the four cells divides again into daughter-cells of unequal size, the 

 result being an eight-celled embryo, each cell with a protoplasmic 

 cap at its dorsal end ((7, D). Next a horizontal division takes 

 place, dividing off the protoplasmic caps as distinct cells/ and so 



Fia. 164. Ovum of Lampetia. nu. nucleus 

 plum, protoplasm ; v. m. vitelline membrane 

 yk. yolk. (After Chun.) 



