ON A NEW SPECIES OF PHYMOSOMA. 171 



Ph. asser; and Ph. nigrescens has smaller diverticula of the 

 same kind. 



The corpuscles contained in this closed system are of two 

 kinds — large clear cells with a well-developed outline, a well- 

 stained nucleus which lies at one side of the cell, and apparently 

 no cell-contents ; the other corpuscles are smaller, with a proto- 

 plasmic body which stains well, and a nucleus in the centre. 



In addition to the fluid of this closed system of spaces, the 

 general fluid of the body-cavity contains corpuscles and the ova 

 or sperm morulse. 



The Brain. 



The nervous system has the same arrangement of nerve- 

 fibres and ganglion-cells as that which I described in Ph. 

 varians. The brain occupies the same relative position, 

 situated at thedorsal side of the lophophore at the base of aslight 

 depression. This depression is much smaller than the similar one 

 in Ph. varians; its area being encroached upon by the bend- 

 ing back of the tentacular crown to form the inner horseshoe, 

 the space is thus rendered rather slit-like and slightly curved 

 (figs. 5 and 8). The depression is lined by a curiously crumpled 

 and deeply pigmented epidermis, with which the brain is in 

 direct continuity in two places. The shape of the brain is 

 different from that of the other species, and this difference 

 corresponds with the alteration in shape of the pigmented area 

 in which it lies. It is bilobed, but the grooves between the 

 lobes are very slight. Each lobe of the brain is smaller in 

 transverse section than those of Ph. varians; on the other 

 hand, their long axis is much longer, so that each lobe is slimmer 

 and more elongated. The narrow outer end of the lobe bifur- 

 cates into two stout nerves, one of which passes round on each 

 side in the connective tissue surrounding the walls of the 

 oesophagus, and fuses with the similar one of the other side to 

 form the ventral nerve-cord. The other passes up into the 

 lophophore in the middle dorsal line, and then turns outwards 

 and runs along the base of the tentacles, giving ofi" a branch to 

 each. There is a second lophophoral pair of nerves of small 



