IV 



PHYLUM CCELENTERATA 



233 



bear a considerable resemblance to the Dicyemidse, but instead of the axial cell 

 there are a number of cells, the ova or sperm-cells. The outer cells are 

 arranged in segments or rings. In front is usually a region composed of a few 

 rings in which the outer cells bear cilia which are directed forwards : then 

 comes a shorter region devoid of cilia, and behind that is the longest region, 

 having cilia directed backwards. In shape the body is usually spindle-like the 

 males (Fig. 178) differing somewhat from the females. In about the middle of 

 the internal space of the male is the compact oval testis containing small tailed 

 sperms. Beneath the outer layer in the male, but not in the female, is a 

 layer of fibres sometimes regarded as muscular. The plasmodia multiply by 

 fragmentation. The development of the embryos either goes on in the intact 

 plasmodium, or the latter breaks up and the embryos are to be found at various 

 stages free in the host. 



In the development of a male from the germ-cell the first segmentation is 

 unequal. The further segmentation results in the formation of a solid morula. 

 The outer cells become differentiated into two distinct groups, the one giving 

 rise to the external layer of the anterior region, the other to that of the 

 posterior region of the body. The inner cells multiply and give rise to the 

 numerous small spermatocytes of the testis. The formation of the layer of 

 fibres only takes place later. 



In the case of the female the segmentation appears to be equal from the first, 

 and results in the formation of a blastula-like stage, which becomes converted 

 into a solid morula-like body by the passing inwards of a number of cells. As 

 in the male, the central cells multiply to form the sexual cells, and the outer 

 cells form the external layer with its segments. In all probability, though this 

 has not been actually proved, the mature sexual animals become free from the 

 plasmodia, and the females, after fertilisation, find their way to another host 

 where they become transformed into plasmodia, the germ-cells of which are the 

 fertilised ova. 



To l)e mentioned in connection with the Dicyemidae and Orthoiiectido 1 , as 

 perhaps allied with them, are the remarkable parasites Amfebophrya and 

 Lolimctnella the former living in certain Radiolariaiis (Protozoa) the latter in 

 the body-cavity of a Fritillaria ( Urochorda), These both resemble the groups 

 described above, and differ from the other Metazoa, in the presence of only a 

 single body-layer. This remarkable simplicity of body-structure occurs also in 

 tiafineUa, though too little is known with regard to this animal to provide 

 adequate data for determining its affinities with certainty. 



Salinella (Figs. 180 and 181), which has only been found on one occasion in 

 water in which some salts from the Argentine Republic had been dissolved, is a 



FIG. ISO. Salinella, longitudinal section. (After Frenzul.) 



minute animal in the form of a somewhat depressed cylinder, open at both ends, 

 and with a wall composed of a single layer of cells. The anterior end is some- 

 what pointed ; around the anterior opening or mouth, which is veiitrally 

 directed, is a circlet of from fifteen to twenty long whip-like cilia. The posterior 

 aperture (ami*), which is usually closed, is surrounded by a few stiff seta?. The 



