DICYEMA, 97 
ular intervals into segments (somites or arthromeres), with 
usually a definite relation of the more important viscera to 
the body-walls—.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 H. 
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 
