v PHYLUM CCELENTERATA 22J 



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. 179) is the phase to which the term 

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

 form embryo*. The latter swim about for a time in the fluid of the kidney of 

 the host ; afterwards they attach themselves by means of a 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 

 is 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 non-development of an enclosing layer, the cells representing the 

 latter being converted into ova. In their development, as in that of 

 the vermiform embryos, the nucleus of the axial cell of the female divides 

 into two. One of these becomes the permanent somatic nucleus of the axial 

 cell : the other gives rise to 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 ova is discharged into the protoplasm of the parent axial 

 cell : this first generation of ova is 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. 180), which were formerly 

 known as the infusorijorm 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 Dicyeina para- 

 are surrounded by the outer cells, which at the posterior doxum. (From 

 end take the form of a flat ciliated epithelium. The com- Ster^olS?^' 

 plete development of the 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 are developed from the fertilised ova and subsequently 

 escape, their development being similar to that of the vermiform embryos : 

 the phase of the parent form to which they are developed is that known as the 

 rJiombogene. After a number of generations of males have been formed in 

 this way, the rhombogene undergoes modification, and the last generation of 

 fertilised ova gives rise, not to males, but to vermiform embryos i.e., to an 

 asexual generation and with these the cycle beeins anew. 



The Heterocyemidce, which are also parasites of the Cephalopoda, resemble 

 the Dicyemidse in most respects, but the head is wanting. 



The family Orthonectidce comprises only two genera Rhopalura and 

 Stcecharlhrum which live as parasites in a Polyclad (Leptoplana), a Nemertine 

 (Linen*), an Annelid, and a Brittle -Star (Amphiura). In the stage that 

 represents the asexual form of the Dicyemidae the Orthonectid assumes the 

 character of a plasmodium, or mass of finely granular protoplasm containing 

 many nuclei, and is capable of active amoeboid movements. In the interior 

 of the plasmodia the sexual forms are developed. A nucleus of the plasmodium 

 surrounds itself with protoplasm and gives rise to a germ-cell, which by a 

 process of segmentation develops into the sexual stage. In some cases only 

 males are developed in one plasmodium and females in another : in others 



