Morse.] . 344 {March 19, 
separating the two halves; in others the ventral cord is distinctly 
separated, but united at each segment by transverse threads, as in 
Sabella, or its two halves may be united by ganglionic enlargements, 
which sometimes correspond to the number of segments. In the 
Hirudine the ventral ganglia are fewer in number than the seg- 
ments. In Aphrodite and Polynoé, according to Grube, there are 
more ventral ganglia than segments. 
It was at one time believed to be typical of the Articulata that 
each segment was characterized by a ganglionic enlargement of 
the ventral cord, and it was supposed to be particularly so with the 
higher Annelids. Claparede calls attention to the fact that this is 
not so in all cases, and especially in relation to the cephalic and thoracic 
ganglia. 
In the Nemerteans the ventral cord is widely separated, and runs 
along each side of the body, and without ganglionic enlargements. 
There appears to be, therefore, various conditions of the nervous sys- 
tem, in which there is in some a simple cesophageal collar; in others, 
a ventral nerve cord, which may be single, or divided into two lateral 
halves, sometimes widely separated, sometimes nearly approximating, 
with, or without, ganglionic enlargements or threads connecting them. 
In some worms accessory pedal ganglia are found. 
In the Brachiopoda we have two lateral ventral cords, widely 
separated, and connected at the esophagus by ganglionic enlarge- 
ments, which send off threads to the pallial membranes, and to the 
various muscles. 
In Lingula these lateral threads seem to be double, connected by 
commissures. In Discina, whose nervous system I have more espec- 
ally studied, the nerve cords are bilaterally symmetrical, and widely 
divaricating. There are no ganglionic enlargements during their 
course to the posterior end of the body, but in their track sending 
off delicate threads, which in Lingula blend with their muscular 
fibres, or pass round the muscles blending with their exterior fibres. 
In Discina these lateral nerve.cords terminate by ganglionic enlarge- 
ments in the last two posterior muscles. These nerve cords were 
correctly interpreted by Cuvier and Owen though mistaken for arte- 
ries by Hancock. This error was corrected by Dr. Gratiolet, in his 
study of Lingula hians;1 and while studying Discina, before be- 
coming aware of Gratiolet’s researches, I found these supposed 
1 Jour. de Conchyliogie. 2 Serie. Vol. Iv, p. 162. 
