TO ARTICULATA. 247 



group of Zoophyta rather than to Mollusca ; but there are others, 

 whose opinion is equally as worthy of attention, who claim for 

 them a place among the latter. As frequently happens, the only 

 basis for the assertion of their zoophytic affinities is that of gen- 

 eral resemblance, no more than you may see in these two figures, 

 one of a Polyp (Metridium, fig. 106) and the other of a Bry- 

 ozoan (Fredericella, fig. 120). If, now, we cover the head of 

 each, the circle of feelers is concealed, and all trace of resem- 

 blance between the two vanishes. It is true that in both there 

 is a free digestive cavity (st) or stomach, properly speaking, but 

 in the Polyp it (fig. 106, st) terminates (atjo) within the great 

 body chamber, so that the food is left to the final assimilation by 

 the general internal surface ; whereas in the Bryozoan there is a 

 complete intestinal canal, (fig. 120, , st, cl 1 ,) within which the di- 

 gestion is carried on exclusively. But what forms a most decided 

 separation is the difference in the relations of the surrounding 

 parts. In the Polyp the organs are repeated like parallel lines 

 along the sides of the body, and have a direct reference to the dis- 

 position of the feelers. This I need not describe in detail, as it 

 has already been done (pp. 57 to 60, and in chap, x.) in former 

 lectures. In the Bryozoan, (fig. 120,) on the contrary, there are no 

 such lateral duplications of similar organs, but everything is de- 

 voted to a unity of purpose ; there is the single retractor muscle 

 (/ 2 ) of the intestine, the single pair of right and left muscles (r, r 1 ) 

 for withdrawing the head within the sheath, (ek,) and the unique 

 nervous ganglion (g) placed exactly on the middle line of the 

 head, all of which find no parallel in the Polyp. 



