496 PBOr. ALLMAN ON THE STRTJCTTJEE AITD 



ttat, on the places where these nuclei lie, the muscular fibre and 

 nucleus are enveloped by a fine sarcolemma. 



Nervous System. — Both Hyatt and Nitsche have made the 

 nervous system of the Phylactolaemata a subject of careful study, 

 and have considerably advanced our knowledge of it. The central 

 nerve-mass is described by Hyatt as presenting a longitudinal 

 depression which indicates a division into two lateral masses, 

 each of which would form a ganglionic centre for the nerves 

 going to its own side of the body. Nitsche has made a similar 

 observation, which thus tends to confirm Dumortier's original 

 view of the existence of two lateral ganglia in the central nerve- 

 mass of LopTiopus, though Hyatt has not succeeded in demon- 

 strating the existence of an oesophageal collar. 



A very delicate oesophageal collar has, on the other hand, been 

 described by JSTitsche, who makes an apt comparison of the 

 central nerve-mass to a signet-ring with two long horns afiixed 

 to the right and left of the stone. The stone represents the 

 ganglionic centre, the remainder of the ring the oesophageal collar, 

 and the two horns are thick chords which pass into the arms of 

 the lophophore. 



The two ends of the ganglion, whose double nature may be 

 inferred from the presence of a deep furrow on the surface which 

 rests on the oesophagus, are continued laterally round the oeso- 

 phagus, thus forming the oesophageal ring just referred to. This 

 ring, however, is very thin and difficult to detect. The two 

 chords which are sent ofi" from the opposite side into the arms 

 of the lophophore are much thicker. 



The central mass is, according to Nitsche, surrounded by a 

 firm envelope, which appears to be identical with the homogeneous 

 membrane already so often mentioned. By means of this the 

 ganglion is attached to the oesophagus, and the horns to the arms 

 of the lophophore. The contents of the envelope consist of a 

 finely granular mass in which very numerous nuclei are scattered, 

 the nuclei preponderating over the finely granular matter both 

 in the ganglion and in the horns. The oesophageal ring, on the 

 other hand, shows an indistinct fibrous structure, and a similar 

 structure is seen in the very delicate peripheral nerves which 

 proceed from the ganglion and horns. 



From the sides of the horns and from their points run a 

 number of fine chords, each of which passes towards the inter- 

 space between every two tentacles, then passes through the walls 



