220 



ZOOLOGY 



SECT. 1\ 



like animal, tiallm.f/u, found in brackish water from the Argentine Republic. 

 Salinella (Figs. KiN and 160) is a minute animal in the form of a somewhat de- 

 pressed cylinder, open at both ends and with a wall composed of a single layer 

 of cells. The anterior end is somewhat pointed ; around the anterior opening or 

 mouth, which is ventrally directed, is a circlet of fifteen to twenty long whip- 

 like cilia. The posterior aperture (anus), which is usually closed, is surrounded 

 by a few stiff setae. The ventral surface is flattened, and is covered with tine 

 vibratile cilia, while on the dorsal surface and the sides are regularly arranged 

 rows of straight setae (non-motile cilia). The internal cavity (enteron) is found 

 to contain sand, plant fragments, and Bacteria ; its surface is beset with lung 

 cilia. Multiplication takes place by transverse fission ; and a process of 

 conjugation followed by encystation has also been observed. 



TRICHOPLAX. 



Another aberrant multicellular animal having affinities with the Protozoa, and 

 not assignable to any of the recognised groups of the Metazoa, is Trichoplax (Fig. 

 170). Trichoplax is a compressed plate-like body of irregular and extremely vari- 

 able shape, but circular in the resting condition, without trace of radial or bi- 

 lateral symmetry. The whole surface is covered with cilia, by means of which the 

 animal performs slow gliding movements, always in close contact with the sub- 

 turn. Frequently it puts forth a process, or more than ' 'ik-h may 



Fii.. iTo. Trichoplax adbaerens, \rn-t f a vurn<-il section. 

 (From Lang, after F. E. ^chnlzc. ) 



extend to a considerable distance and assume a variety of -hapes. There art- 

 three distinct cell-layers an upper epidermis, consisting of flattened cells closelv 

 united by their edges so as to form a syncytium like that of the ectoderm of 

 Sponges ; a lower epidermis, composed of pyramidal cells, the apices of which are 

 produced and unite with the elements of the middle layer ; and a middle layer 

 of spindle-shaped or slightly branched cells, which anastomose to form a network 

 containing in its meshes hyaline, fluid ground-substance. In addition to their 

 nuclei, these cells of the middle layer enclose several kinds of highly refracting 

 granules and spherules. There are no stinging-cells, and no trace of internal 

 organs of any kind. Little hyaline papilla- which appear round the edges may 

 be for temporary fixation. Multiplication by simple fission has alone been 

 observed. 



The position of Trichoplax and of the nearly allied T,\jnoj>lax is quite 

 uncertain. It cannot be a larval form, as it lias been kept under observation 

 for nearly a year without exhibiting any change. It is possible that both it and 

 S<i/'in<Jla, neither of which have been observed in quite natural circumstances, 1 

 maybe abnormal modifications brought about by the influence of exceptional 



CO i : 



1 Trichoplax has only been observed in marine aquaria ; Salinella only in 

 water in which salts from certain saline springs in South America had been 

 dissolved. 



