26 



INTRODUCTION TO CLASSIFICATION. 



general substance of the body, and provided with permanent 

 apertures, as in the Hydrozoa and Actinozoa, but completely 

 shut off from the perivisceral cavity, and in direct communica- 

 tion only with the external medium. All the Polyzoa possess 

 a nervous system, the characters and position of which are 

 very well defined. It consists of a single ganglion (Fig. 9, w)> 

 placed between the oral and the anal apertures, and sending 



Fig. 9. 



Fig. 9. Plumatella repens, a single cell more magnified ; m, calyx at the base of the 

 ciliated tentacula borne by the disk or lophophore ; k, gullet ; g g, stomach ; h, in- 

 testine; t, anus; w, nervous ganglion (after Allman). 



off nerves in various directions. It has been affirmed that, 

 in some Polyzoa, there is a more extended system of nerves by 

 which the various zooids of the compound organism are placed 

 in communication ; but of that we want further evidence. In 

 these animals no heart has been discovered as yet, the matters 

 which result from digestion percolating througli the walls of the 

 intestine, and becoming mixed with the perivisceral fluid. One 



