Japanese Tricladida Maricola. O^ 



least six in number, throughout their whole lengtli, straight from 

 trunk to trunk. Unlike the preceding two species there exist no 

 three special transverse commissures in the brain region. From 

 the main trunks, at points of insertion uf tlie transverse commis- 

 sures in the trunks, are given off lateral nerves (îil), which join the 

 marginal nerve plexus (^nm). The dorsal longitudinal nerve trunks 

 appear to take their origin from the marginal nerve at points of 

 exit for the ventral anterior longitudinal trunks, and pursue a 

 backward course directly below the sheet of the dermal muscula- 

 ture. The}^ are connected together by a number of transverse 

 commissures and with tlie marginal nerve l)y the same number 

 of lateral nerves, cpite similar in appearance to tlie ventral 

 anterior longitudinal nervous system. 



Crowded around the fibrillous mass of the brain are numerous 

 large nuclei containing ganglionic cells easily stainable. The cel- 

 lular coating is generally much thicker on the ventral than on the 

 dorsal side. The sensory nerves are also seen with their setting 

 of ganglionic cells, which are generally either unipular or more 

 commonly Iji- or multipolar. Each brain mass is invaded b}" a 

 peculiar tissue well known as ' ' Substanzinsel ' ' , which consists 

 for the most part of the ganglionic cells l)eset with connective 

 tissue and is traversed l)y some dorso- ventral muscular fibres. 



Posterior Longitudinal Nervous System. — As usual, there exist 

 three pairs of longitudinal nerve trunks, viz., ventral {nt)^ dorsal 

 and lateral {nl). Of these the ventral trunks are the most import- 

 ant and are developed to a considerable extent. Behind the brain 

 they generally proceed, running nearly parallel to each other, until 

 near the copulatory organs and then gradually approach to each 

 other. Only in Ect. limuU, however, they are separated in the 

 region of the copulatory organs from each other, because of the 

 remarkable enlargement of the vestibulum. The longitudinal 

 trunks are connected together by a numljer of transverse commis- 

 sures, straight from trunk to trunk. S()metimes with, sometimes 

 without brandling in their passage, much like a rope ladder. 

 Some of the commissures are much more well developed than 

 -others and these most frequently occur immediately behind the 



