1889.] 



On Phymosoma varians. 



125 



of them have at their free end a bubble or vesicle in which these 

 grannies have accumulated. From time to time these vesicles break 

 oft and lie in the lumen both of the secretory part and of the bladder, 

 and are no doubt extruded from the body. The whole process is very 

 like the excretion of milk in a mammary gland. The only other 

 structures, besides these vesicles, found in the lumen of the nephridia 

 are the generative cells, ova and spermatozoa ; it is remarkable that 

 the ccelomic corpuscles never enter them. 



The Nervous System. — The brain is a bilobed organ lying imme- 

 diately beneath the pre-oral lobe, with the ectoderm of which it is 

 continuous. The ventral and posterior surfaces project into a blood 

 sinus which is situated in the neighbourhood of the junction of the 

 lophophoral vessels with the dorsal vessel. The ganglia in the brain 

 form a cap on the anterior, dorsal, and posterior surfaces, enclosing 

 the fibrous tissue which comes to the surface of the brain ventrally. 

 The ganglion cells are mostly small and bipolar, but on the posterior 

 surface are a certain number of unipolar giant ganglion cells, with a 

 diameter at least four times that of the smaller cells. 



The brain gives off three pairs of nerves ; at the side a pair of 

 circumcesophageal commissures which pass round the oesophagus and 

 fuse to form the ventral nerve cord. This has no trace of a double 

 origin or of segmentally arranged ganglia, but from time to time it 

 gives off a nerve which passes into the body wall and there splits into 

 a right and left nerve ; these reunite in the dorsal middle line and so 

 form a. nerve ring. These nerve rings are especially conspicuous in 

 the skin of the introvert. The second pair of nerves given off from 

 the brain pass into the base of the lophophore, and give off a branch 

 into each tentacle, where it lies immediately beneath the ciliated 

 groove. The third pair, which arise nearest the median line, pass to 

 supply the skin of the pre-oral lobe. 



The sense organs are of two kinds : (i) sensory pits in the brain, 

 (ii) ectodermal sense organs in the introvert. The former are two 

 pits which open on to the pre-oral lobe, and end blindly in an ex- 

 panded vesicle in the substance of the brain. .They are lined with 

 columnar epithelium, which at the inner end is crowded with dark- 

 brown pigment. Their lumen sometimes contains a clot. The latter 

 are groups of ectodermal cells, which have increased in size and at 

 their outer end are provided with short stiff processes which project 

 above the general level. These are gathered together into a small 

 brush by a chitinous ring which surrounds their base. These organs 

 occur in rings at the base of the rings of hooks in the introvert. 



The Reproductive Organs. — Phymosoma varians is dioecious. In both 

 sexes the reproductive organs form fimbriated ridges which are 

 attached to the bases of the ventral retractor muscles, and are con- 

 tinuous across the interspace between these two muscles, ventral to 



