246 SUMMARY OF CURRENT RESEARCHES RELATING TO 



geneous ground-substance being covered by a very tbin investment. 

 Tbe muscles wbich move the ventral setse are inserted into this outer 

 sheath, and these muscles are to be distinguished by their relation and 

 characters. In addition to these ventral setae, which are formed in all 

 the Echiuridae, there are others which are peculiar to the genus 

 Echiurus. They form a group of from thirteen to sixteen, which are 

 arranged in two rows around the anus, and may consequently be known 

 as the anal setae. They each consist of a straight shaft which, like 

 that of the ventral setse, lies in the body, and of a straight or slightly 

 curved projecting hook. These would api^ear to have, as the ventral 

 setfe certainly have, a locomotor function ; but it is as yet impossible 

 to define more exactly the role that they play. After a description of 

 the reserve set of setae the author passes to the 



Nervous System. — The central portion is formed on the type of Bo- 

 nellia, as described by Lacaze-Duthiers. Along the middle ventral line 

 of the coelom there runs a ventral medulla, which divides anteriorly into 

 two processes which are connected by a dorsal supra-oesophageal arch. 

 The ventral portion is, in the adult, an almost cylindrical cord, made up 

 of cellular and fibrous elements ; the former chiefly occupy the sides and 

 part of the dorsal region, while the latter form the internal and the ven- 

 tral portion. In cross section the ganglionic masses exhibit, therefore, 

 a well-marked bilateral symmetry, and are more closely approximated 

 to one another in the dorsal than in the ventral parts. The relations 

 of the cells to the fibres are very difficult to detect, and of the fibres 

 many are certainly of other than a fibrous nature. The true nerve- 

 fibres would seem to be arranged in regular bundles. It is of great 

 interest and importance to note that in the young the arrangement of 

 the ganglionic cells is less regular. They do not form an uninter- 

 rupted and regular band on either side, but are found in greater 

 quantities at certain points, and are for short spaces almost completely 

 absent. Thus, then, they call to mind the arrangements which obtain 

 in the Annelides. The reality, of the bilateral symmetry is best 

 spoken to by the relations of the peripheral nerves ; those of either 

 side always exactly correspond. The remarkable arrangement de- 

 tected by Keferstein and Ehlers in Sipuncuhis has also been observed 

 in Echiurus. Each nerve runs for a short space through the coelom, 

 and then passes almost perpendicularly through the inner and median 

 muscular layers, whence it runs parallel to and between the circular 

 muscles and the longitudinal layer towards the back, where it unites 

 to form a circular ring with its fellow of the opposite side. A some- 

 what problematical structure is to be seen in transverse sections of the 

 ventral cord. This is the so-called longitudinal canal, which lies just 

 below the dorsal middle line, and which in preparations is often filled 

 with a coagulum. It is easy to compare it with the similarly pro- 

 blematical structures found in Annelids and known as the giant nerve- 

 fibres of Leydig, the tubular fibres of Claj)arede, or the neural canal 

 of M'Intosh. The latter name is adopted by Dr. Spengel for the 

 cavity found in Echiurus. Anteriorly it passes into the oesoj)hageal 

 commissures. Some attempts have been made to compare this struc- 

 ture with the central canal of the Vertebrate spinal cord, but the 



