NEMER TINES. 2 6 1 



There is no true coelom in the Nemertines, and the 

 space between the alimentary canal and body-wall is oc- 

 cupied by a gelatinous mesenchyme, containing muscular 

 and connective tissue elements. In Balanoglossus the cav- 

 ity of the ccelom becomes largely obliterated in the adult, 

 by the proliferation of cells from the epithelium of its 

 walls, thus filling up the cavities with a more or less solid 

 parenchymatous tissue. 



Like Balanoglossus, the Nemertines have a straight ali- 

 mentary canal, provided with paired lateral outgrowths or 

 intestinal coeca, and a terminal anus. 



The gonadic sacs of the Nemertines offer a striking re- 

 semblance to those of Balanoglossus. They occur as a 

 metameric series of paired sacs, which alternate with the 

 above-mentioned intestinal cceca, and communicate with 

 the exterior by short tubes, which are at first solid, as in 

 Balanoglossus, subsequently becoming hollowed out and 

 opening above the lateral cords (Fig. 124). 



Finally it should be pointed out' that, while excretory 

 organs, in the form of a well-developed single pair of 

 elongated nephridia, provided with numerous internal 

 "end-sacs," are present in the Nemertines, nothing of the 

 kind has yet been detected in Balanoglossus. 



CEPHALODISCUS AND RHABDOPLEURA. 



It is interesting to note that there are some remarkable 

 animals which stand in a similar relation to Balanoglossus 

 that the Ascidians do to Amphioxus. While Balano- 

 glossus is free-living, does not produce buds, and has a 

 straight alimentary canal, these creatures, of which only 

 two genera are at present known, Cephalodiscus and Rkab- 

 doplenra, lead a sessile existence, produce buds, and have 



