6 T. H. MORGAN. 



and proximally a prolongation of the body which is to be 

 regarded as the basal segment of the appendage. Between the 

 first pair of appendages the proboscis projects forwards, with 

 the mouth opening at its distal end. Behind the base of the 

 proboscis is seen the pair of ventral ganglia. From these the 

 circumcesophageal commissure ascends upwards around the 

 oesophagus, and laterally are given off on each side a pair of 

 nerves to the second and third pairs of appendages. Dorsal to 

 these ganglia — four in all, the two pairs being fused — is seen the 

 outline of the mesenteron, which ends blindly behind. About 

 the middle of the embryo and just back of the pair of ganglia is 

 seen a pair of oblong masses, which are the beginnings of another 

 pair of ventral ganglia. In the basal joint of the first pair of 

 appendages is an opaque mass composed of large cells, from 

 which runs out a duct towards the base of the spine, and Dohrn 

 has traced it out to the very tip of the spine, where it opens to 

 the exterior. This organ is a gland, and its duct is the long 

 tube in the spine. 



A dorsal view of the embryo shows the brain, just above and 

 a little in front of the ventral ganglia. On its upper surface are 

 a pair of small pigmented eyes. A transverse section through 

 the body of this Pantopod-larva is shown in Plate I, 13. It 

 passes through the center of the body and cuts below the large 

 ventral ganglionic mass, passing through the base of the third 

 pair of appendages. Above it cuts the posterior part of the 

 brain. In the center of the section is the digestive tract. This 

 is cut at a point where it is about to give off diverticula to the 

 first and third pairs of appendages. The first of these is 

 marked D 1 and the second D n . 



Below the digestive tract is the second pair of ganglia, which 

 sends out nerves to the corresponding appendages. Above the 

 digestive tract the section at B cuts the posterior part of the 

 brain. The cavity of the body has a few scattered mesoblast 

 cells in it and a few bits of broken muscle fibers, These fibers 

 in the living embryo served to connect the mid-gut to the body- 

 walls. In the base of the legs are seen several cells, which at M 

 are arranged as though around a central cavity. This cavity does 

 not seem to connect with the mid-gut, and it seems very probable 

 that it represents the body-cavity in the legs, and that the sur- 



