ON THE DIPLOCHORDA. 345 



which bear a remarkable resemblance, both in appearance and 

 structure, to the *' branchial organs " found in the sedentary 

 Annelids, such as Pot a mi 11 a and Sabella. The figures and 

 description of these organs as given by Andrews (3) (and a 

 comparison of them with these organs of Cephalodiscus) 

 can leave little room for doubt that the functions in each case 

 are similar. It seems most reasonable to regard them tenta- 

 tively as primitive eyes, though the presence of compound 

 eyes in the Chordata is rather remarkable. 



Andrews conducted experiments with Pot am ill a to deter- 

 mine if the "branchial organs" were possibly phosphorescent 

 organs, but apparently with a " negative result." The pre- 

 sence of hairs (sensory ?) in the branchial eyes of some of the 

 Annelida, such as My xi cola and Filigrana, led him to 

 suspect a possible sensory function other than that of vision, 

 but there is no indication of hairs or cilia in the branchial eyes 

 of Cephalodiscus. 



Branchial Plumes (Lophophore). — The collar region, as 

 is well known, is produced on each side into six arms or 

 plumes, and each of these has a double row of smaller pinnae, 

 borne laterally. 



The structure of a plume is very similar to that of the rest of 

 the collar region. A layer of epithelial cells rests upon the meso- 

 blastic chondroid tissue (PI. 26, fig. 20), which encloses a well- 

 defined cavity. Across this are stretched many fine strands of 

 protoplasm, with nuclei dotted here and there in their course. 

 The transverse section of a plume is crescentic in outline, and 

 the dorsal side is convex. The epithelium on this side is 

 thinner than that on the concave ventral surface, and consists 

 of closely set cubical cells ; it is continuous with the epithelium 

 of the dorsal collar region. In the mid-dorsal line a flattened 

 nerve runs throughout the length of the plume, terminating 

 distally in the " branchial organ, '^ and joining proximally with 

 the other five to enter the nerve-ganglion. Immediately internal 

 to this nerve (in the plume) runs a main blood-vessel, usually 

 somewhat triangular in transverse section (fig. 26), and con- 

 tinuous with the main dorsal blood-vessel in the collar region. 



