22 ARTHUR E. SHIPLEY. 



lying dorsal to the ossophagus. This communicates with the 

 lophophoral sinus in the dorsal middle line. Just at this point 

 is a blood-sinus which surrounds all those parts of the brain 

 which are not continuous with the ectoderm. This system of 

 blood-vessels is closed. It contains numerous small oval 

 corpuscles. In addition to these the cnelomic fluid contains a 

 number of much larger corpuscles, as well as ova and sperm 

 morulsB. The coelom is lined by a fiat epithelium which is not 

 ciliated. 



(9) The brain is a bilobed mass, partly connected with the 

 ectoderm of the praeoral lobe and partly surrounded by a 

 blood-sinus. The relative position of the ganglion-cells and 

 fibrous tissue is described above. There are a number of 

 giant ganglion-cells arranged in the lateral and posterior 

 parts of the brain. 



(10) The brain gives off three pairs of nerves : (1) the first 

 pair pass to supply the pigmented tissue of the prseoral lobe ; 

 (2) the second pair run along the base of the lophophor, and 

 send a branch into each tentacle ; (3) the third pair pass 

 round the oesophagus, and unite to form the ventral nerve- 

 cord. This is supported by a strand of muscle in each side, 

 and by numerous connective-tissue strands which pass to the 

 body wall. It has no trace of a double structure, and no seg- 

 mentaily arranged nerve-ganglia. It gives off from time to 

 time a median nerve, which soon splits, and each half runs 

 round the body, these fuse together again in the dorsal 

 middle line, thus forming a nerve-ring. 



(11) The sense-organs consist of two pigmented pits in the 

 brain, and of certain structures in the introvert. The former 

 pits open on to the prseoral lobe, and then pass into the brain at 

 each side. Each pit is bent on itself, and expands slightly at 

 its inner end. The cells lining the pit are crowded with black 

 pigment. The sense-organs on the introvert lie in rows close 

 behind the i^ows of hooks. Each consists of a number of ecto- 

 dermal cells produced outwards into a stiff process. These 

 processes are gathered up into a little brush by a chitinous 

 ring which surrounds their base. 



