IV. Ophiuroidea. 15 



ganglia corresponding to the vertebrae, and communicating with one another 

 laterally by commissures above the blood-lacuna. On the ventral side between 

 the vertebrae there is but one layer of cells, which are partly ganglionic and 

 partly supporting cells. The latter are absent in the vertebral regions, where the 

 multipolar ganglion cells are in many layers. In Opkioglyplui albida three sets of 

 nerves come off on each side of the radial band. Nervus late rails I is formed 

 by fibres derived from the median and dorsal portions between the ganglia, together 

 with ganglion cells from the ventral layer. It surrounds the foot, forming a pedal 

 ganglion from which fibres run down in the outer wall of the schizocoel, to join a 

 chain of paired ventral ganglia beneath the median schizocoel. A second nerve 

 leaves the pedal ganglion, to enter the foot, and a third passes up towards the 

 dorsal surface Avhere its branches join those of its fellow on the other side of the 

 arm ; while a fourth nerve enters the vertebra, partly ramifying there, and partly 

 passing on, to enter an intervertebral ganglion. Nervus late rails II is also 

 derived from both median and dorsal portions of the radial baud in the region of 

 a ganglion, supplying the integument and the musculature of the water-vessel. 

 N ervus later alls III comes off near the end of the ganglionic region and also 

 supplies the integument of the side and lower arm plates. Both in the dorsal and 

 in the ventral integument the intervertebral regions are those most richly supplied 

 with nerves. The nerves supplying the great muscles of the arms are derived 

 entirely from the ganglia on the dorsal side of the radial band, some of their 

 libres entering the intervertebral ganglia. The bursal nerves arise from the pedal 

 ganglia of the second buccal feet. In the oral ring there are no supporting cells 

 and the nerve cells are of two sizes, the smaller ones being placed more peripher- 

 ally. The ventral layer of cells shows a ganglionic arrangement which is absent 

 dorsally. The oral ring supplies nerves to the ten buccal feet, and to the two 

 sets of interradial muscles. The two latter nerves each form a small ganglion and 

 come off from a common trunk which is derived from both the dorsal and the 

 median regions of the oral nerve ring. Ten nerves also leave it, to enter the wall 

 of the gullet, and ramify among the basal ends of the epithelial cells. The tube- 

 feet of Ophiothrix fragilis bear small processes which are regularly arranged in 

 rows, and consist of groups of elongated cells, the basal ends of which pass into 

 nerve fibrils. These join a nerve ring which is formed by lateral extensions from 

 the main pedal nerve. The segmentation of the ventral part of the nerve band is 

 very marked in this type, and the nerve cells are of two sizes with well developed 

 supporting fibres. Lateral nerves leave it between the ganglia and enter the side 

 plates, where they give oft' numerous branches, each bearing a small ganglion and 

 entering a spine. At the end of the arm the nerve baud is very distinctly seg- 

 mented and it passes into a ganglion at the base of the terminal tentacle, while 

 it also sends branches to the musculature of the booklets. - - The schizocoel spaces 

 of the arms are quite unconnected with the coelom, which is lined by ciliated, 

 cubical, or flattened cells. But the mid- dorsal line of the arm is marked by a 

 band of larger cells with longer cilia. In Opkioglypha albida there is (or may be) 

 on the dorsal surface a supplemental ciliated water-pore, which leads directly from 

 the exterior into the coelom. A tubular sinus, belonging to the schizocoel system, 

 encloses the axial organ, together with the water-tube, which winds in a spiral 

 before entering the madreporite, where it forms no ampulla in 0. albida. This 

 type has two water-pores, the canals of which join, and the single one thus formed, 

 after giving off lateral extensions, which end blindly, is continued into the water- 

 tube. The water-vascular ring communicates with Kohler's oral space by 

 minute openings governed by sphincter muscles. The lip also contains a schizocoel 

 space which communicates with the oral schizocoel ring. This is connected with 



