Oatty Marine Laboratory^ St. Andrews. 5 



is differentiated on the dorsum in front of the aperture. 

 Both dorsal and ventral surfaces are plano-convex, thus in 

 transverse section heing more or less elliptical, the dorsal, 

 however, being generally more smoothly rounded, with a 

 pale streak in the middle line from the dorsal vessel. The 

 ventral surface has a continuous series of scutes from one 

 end to the other, and, with the exception of the first, all are 

 divided by the median groove, giving a right and a left 

 scute to every segment. The number of segments varies 

 from 200-270 or more. 



The first region oF the body (the so-called thoracic region) 

 is distinguished b}' the direction of the bristle-tufts, which 

 slope upward and backward, and by the presence of ventral 

 rows of hooks. The setigerous processes of this rej^ion vary 

 from seven to eleven pairs, one side occasionally having nine or 

 ten and the other ten or eleven. The first setigerous process 

 arises immediately behind the dorso-lattral lamella, and is 

 sraalli^r than those following; moreover, it has no row of 

 hooks dorsally. The bristles are arranged in a rather dense 

 group and show a longer and a shorter series. The longer 

 forms have cylindrical striated shafts, slightly narrowed toward 

 the orij^in of the wings, which are narrow, the bristle there- 

 after ending in a translucent and somewhat strong though 

 flexible tip. No serrations on the edges of the Avings have 

 been seen in these, though strise go to the edge. The bulk 

 of the gioup of bristles is made up of the^e with shorter 

 shafts and broader wings, the whole tip being rather broad 

 until near its extremity, where it is somewhat abruptly 

 tapered to a fine point. In the cluster of bristles in the tuft 

 various stages occur in the developing bristles — some re- 

 sembling a long narrow knife-blade, others a deep-bellied 

 shorter blade. The succeeding tufts are of similar sha[)e 

 (that is, somewhat tiattened), but they are longer and stronger, 

 and the edges have distinct serrations. The anterior seti- 

 gerous processes form a somewhat flattened cone, the base 

 ventrally b.ing prolonged into a prominent ridge, bearing 

 the hooks which lie between two raised margins, constituting 

 a narrow flap posteriorly in each segment. Moreover, a 

 distinct papilla occurs on the anterior edge of the tip. 



The setigerous processes of the second region form stiff 

 narrow cones which project nearly straight outward, the 

 bristles only being visible at the tip. A soft and rather 

 swollen process bearing the hooks lies above their dorsal 

 edges. The bristles form a kind of pillar, narrow at the 

 base and en'arging at the tip to about double the diameter 

 at the base of the wings, which give to the distal end a 



