200 BULLETIN OF THE 



into a clear space beneath the ventral surface, the beginning of the oesophagus. 

 No ciliary ring lias yet appeared at the anal end of the larva. 



The embryo, although possessing spines which can be projected from the 

 body, and a ring of cUia about the head, is still with others enclosed in the 

 mass of slime in which they were laid. All have very limited movements, 

 even when artiflcially set free from the cluster in which they are bound 

 together. Their bodies, especially the central part, have a dirty green color, 

 while the head, ciliated band, and body walls are more transparent. 



In the subsequent changes which the external form of the larva passes 

 through, there is little of interest to be mentioned. Parapodium after parapo- 

 dium is added, increasing the length of the body of the worm. The new seg- 

 ments always arise back of that which is previously formed, while in advance 

 of all, between the first body segment and the posterior border of the ciliated 

 band, marked on the sides by the lateral rows of pigment spots, there are no 

 lateral appendages to the external body walls. From its position and its sub- 

 sequent history, this bare part of the body immediately behind the head is 

 probably homologous with the anterior bodj^ region (ar. Fig. 1), which has 

 been already elsewhere described in the genus Prionospio. 



In my figure of the larval Lumbriconercis witb two pairs of well-formed 

 spines, it will be noticed that the digestive tract has already diff'erentiated itself 

 into two regions, an anteiior, larger, and more capacious, and a posterior more 

 tubular portion. The former of these is later changed into the stomach, while 

 the latter is destined to form the intestine. The point of separation of one of 

 these from the other is not yet well marked. 



At the anterior end of the stomach, on one side, a globular body is constricted 

 from the stomach walls at about this time. The ultimate history of this organ 

 has not been traced, and its function is unknown. A similar body has been 

 mentioned in the young Nephthys. The jaws in the larval, as in the adult 

 Lumbriconereis, are very complicated, and consist of two parts, a dorsal and a 

 ventral. The ventral jaw is formed of a single crescentic chatinous plate, the 

 horns of which extend forward. The regularity of the concavity of the ante- 

 rior edge of the jaw is broken liy a single small median tooth. The posterior 

 edge of the jaw is continued backward into two elongated projections, which 

 extend parallel with each other, and are separated by a narrow slit. 



The dorsal jaws are still more complicated than the ventral. They consist 

 of four pairs of chajtinous articulations which act as teeth, and are arranged in 

 pairs the members of which are placed opposite each other. The two anterior 

 of these are simple teeth with smooth edges, the former with a triangular out- 

 line ; the latter is more elongated, narrower, and more ])ointed. 



The l)0(ly of the dorsal jaws is formed of two flat or slightly curved oblong 

 plates, whose opposite edges are serrated. By the approximation of these bor- 

 ders, the true function of this complicated mechanism is accomplished. To 

 these ol)long, serrated plates, on their posterior border, are articulated the last 

 pair of bodies Avhich fonii the dorsal jaws. These take a triangular shape, and 

 have for their function a firm attachment for the remaining parts of the jaw. 



