iv PHYLUM CCELENTERATA 231 



condition ventral and dorsal sides are distinguishable. It consists of a swollen 

 disc of four cells and a ring of four or five yole cells. The cells of the head all 

 bear cilia, which are shorter and thicker than those of the body-cells. 



The body consists of a single large axial cell, and of a single layer of outer 

 cells which completely invest the axial cell. The outer cells which follow 

 immediately on the head are distinguishable from the rest by their granular 

 contents, and by their being dilated internally in such a way that the apex of 

 the axial cell is constricted. 



The axial cell is either almost completely cylindrical or spindle-shaped, and 

 is covered in its entire extent by the outer cells. It presents a differentiated 

 cortical layer, beneath which the finely granular gelatinous contents are at first 

 homogeneous, but afterwards become vacuolated. In the middle of the cell is a 

 large oval or ellipsoidal nucleus. 



The life-history is a true alternation of generations. The primitive nucleus of 

 the axial cell divides, mitotically to form a smaller asexual germ-nucleus and 

 a larger nucleus the definitive nucleus (somatic nucleus) of the axial cell. 

 Further germ- nuclei result from subsequent divisions. The germ-cells undergo 

 a process similar to segmentation. Of the cells thus formed one gives rise to 

 the axial cell of the embryo : the others, increasing in numbers and becoming 

 smaller, gradually grow over the axial cell until they at length completely 

 enclose it. The embryo increases greatly in length, and escapes from the 

 interior of the parent, which it completely resembles, by perforating the body- 

 wall. This phase of the parent animal (Fig. 176) is the phase to which the term 

 nematogene is applied : the asexually developed young are the so-called vermiform 

 embryos. The latter swim about for a time in the fluid of the kidney of the 

 host ; afterwards they attach themselves by means of the head to certain 

 appendages the venous appendages of the walls of the cavity. A number of 

 generations of these asexually developed forms succeed one another until, when 

 the venous appendages of the kidney have become thickly infested with the 

 parasites, a change takes place, and a sexual process of multiplication becomes 

 initiated. In the interior of the nematogene a change is observable, and female 

 sexual individuals are formed instead of vermiform embryos. Unlike the latter 

 the former do not leave the body of the parent ; they also differ in the 11011- 

 developmeiit of an enclosing layer. In their development, as in that of the 

 vermiform embryos, the first nucleus of the axial cell of the parent divides into 

 two. One of these becomes the permanent somatic nucleus of the axial cell : the 

 other becomes the nucleus of the primitive ovum, and surrounds itself with 

 protoplasm. This divides to give rise to a number of ova, which become more 

 numerous till they come to fill the axial cell. Then the first generation of these 

 ova are discharged into the protoplasm of the parent axial cell I this first 

 generation of ova are derived from the cells representing the outer layer. 

 Later further generations of ova, which are the descendants of the primitive ova, 

 make their escape. These all eventually wander away into 

 the protoplasm of the axial cell of the parent, increase in 

 size, undergo a process of maturation, and become fertilised. 

 Fertilisation is effected by means of typical tailed sperms 

 developed in a second set of sexual individuals, the males 

 (Fig. 177), which were formerly known as the infusoriform 

 embryos. In its mature form the male is approximately 

 pear-shaped, the narrower end being posterior. Several 

 axial cells are present : these form the testes, in which the 



sperms are developed. They are surrounded by the outer Fl ;. ] ' ' ~ 



n 1-1 . , re Dicyema ^.. 



cells, which at the posterior end take the form of a flat doxum (From 



ciliated epithelium. The complete development of the Broim's_ Thien; 

 sperms only takes place when the young male leaves the 



host in which it was formed and seeks a new one ; thus it is only by the 

 sperms of a male from another host that the ova can be fertilised. The males 



