64 INVERTEBRATE MORPHOLOGY. 



1.5 to 4 mm., but capable of great alteration of form. It is' 

 flattened, and creeps about upon the walls of the aquaria in 

 an amoeboid manner. It consists, however, of numerous cells 

 (Fig. 30, B), the upper surface being covered by a flattened 

 ciliated epithelium, and the lower formed by a layer of 

 columnar cells also ciliated, while the space between the two 

 surfaces is occupied by a network of branching cells, the 

 branches appearing to unite with those of adjacent cells and 

 with prolongations from both the upper and the lower epithe- 

 lium. The arrangement suggests the three germ-layers ecto- 

 derm, endoderm, and mesoderm, but until more is known con- 

 cerning the reproductive processes such an homology is 

 unwarranted. At present the organism is only known to re- 

 produce by division, and no structures have been discovered 

 which may be identified as ova or spermatozoa. Beneath the 

 upper epithelium, imbedded in the cells of the middle tissue, 

 large refractive spheres (Fig. 30, B, r) and yellowish-green 

 botryoidal masses (h) occur, but they have apparently no con- 

 nection with reproduction. 



The DicYEMiDa;. 



The Dicyemidse are elongated vermiform organisms which 

 are parasitic in the renal organs of the Cephalopods. The 

 various species of Dicyema (Fig. 31) vary in length from 

 0.5-7 mm. and are all very simple in structure, consisting of 

 a single elongated central cell (Fig. 31, G) extending from one 

 end of the body to the other and covered by a number of 

 ciliated cells arranged in a single layer. Some of these, situ- 

 ated at one end of the body, are smaller than the others and 

 mark off the anterior extremity ; there is no mouth or diges- 

 tive tract and no sense-organs. 



Eeproduction is carried on by the development of germ- 

 cells (g) produced by the division of the nucleus of the central 

 cell and the concentration around the nuclei so produced of a 

 portion of its protoplasm. The development of these germ- 

 oells is apparently parthenogenetic and no male Dicyema is as 

 yet known. In young individuals the germ-cells segment in 

 the interior of the central cell and give rise to " vermiform " 



