82 



ZOOLOGY 



are simpler in their organisation, Mesostoma, is, however, not 

 infrequently found in ponds and streams, and the transparency 

 of its body permits a more thorough examination of its internal 

 organs than is the case with most of the members of the group. 



Mesostoma Hhrenbergii is a flat leaf-shaped Ehabdocoel, 

 its dorsal surface being but very slightly vaulted. Its length 

 does not exceed 15 mm., and its greatest breadth 5 mm. 

 Another member of this genus has a square cross-section, 

 with the angles somewhat produced. The body is very trans- 

 parent, and sometimes nothing of it can be seen but the brown 

 stomach and occasionally the red eggs. 



The epidermis is composed of flat cells, with irregular out- 

 lines and conspicuous nuclei. The whole surface of the body 

 is covered with fine cilia, somewhat longer on the ventral side 

 than on the dorsal. Within the layer of ectoderm cells, but 

 separated from it by a basement membrane, is a thin sheet of 



Fig. 56. — Integument of 

 Mesostoma lingua, 0. Soh. 

 After Von Graff. 



1. Epidermis with perfora- 

 tions (2) through which 

 the rhabdites (3) project. 

 Beneath is the basement 

 membrane (4), and be- 

 neath this again the 

 muscular layers consist- 

 ing of circular (5), longi- 

 tudinal (6), and diagonal 

 (7) fibres. 



circular muscle fibres, and within this a layer of longitudinal 

 fibres, crossing the outer layer at right angles, and a few 

 oblique fibres (Fig. 56). These tissues form the integument, 

 and surround a mass of tissue, the parenchyma, in which the 

 various organs of the body are embedded. The coelom is 

 broken up into irregular spaces or splits by the presence of 

 numerous dorso-ventral muscle fibres and connective tissue 

 strands. The lacunae left between these branching fibres 



