DIOTEMA. 139 



ular intervals into segments (somites or arthromeres), with 

 usually a definite relation of the more important viscera to 

 the body -walls — i.e., a digestive tract extending from the 

 head to the end of the body, the nervous system consisting 

 of a brain, or supracesophageal ganglion, and a single or, 

 more commonly, double chain of ganglia, resting on the 

 floor of the body ; a dorsal vessel or heart is usually present 

 being situated above the digestive tract. True jointed 

 appendages are never present, and in the embryo the 

 blastoderm is usually without any " primitive streak " (the 

 Annulata excepted). This definition will exclude the worm- 

 like Actinozoa and Holothurians. 



Before describing the lowest class of worms, we may call 

 attention to a small aberrant group called Mesozoa by E. 

 Van Beneden, the position 'of which is doubtful, though the 

 animals composing it are probably aberrant worms. 



In 1830 Krohn observed in the liquid bathing the " spongy 

 bodies," or venous appendages, of different species of 

 Cephalopods certain filiform bodies, covered with vibratile 

 cilia, and resembling Infusoria. They were afterward named 

 Dicyema by Kolliker, who with others considered them as 

 intestinal worms. In 1876 Professor E. Van Beneden gave 

 a full account of their structure and mode of development. 

 He states that these organisms have no general body-cavity, 

 but that the body consists (1) of a large cylindrical or fusi- 

 form axial cell, which extends from the anterior extremity 

 of the body, which is slightly enlarged into a head, to the 

 posterior end ; (2) of a single layer of flat cells forming 

 around the axial cell a sort of simple pavement epithe- 

 lium. All these cells are placed in juxtaposition like 

 the constituent elements of a vegetable tissue. There is 

 no trace of a homogeneous layer, of connective tissue, of 

 muscular fibre, of nervous elements, nor of intercellular 

 substance. There is only between the cells a homoge- 

 neous substance, such as is found between epithelial 

 cells. The axial cell is regarded as homologous with the 

 endoderm of the higher animals (Metazoa). Van Beneden 

 designates as the ectodermic layer the cells surrounding the 

 large, single axial cell. There exists no trace of a middle 



