Plan of Polyzoon. 237 



Figure 4. 



Plan of Polyzoon, from Bronn's ' Klassen und Ordnungen des Tliierreiclis,' iii. I, 

 Taf. xviii., fig. i. after Allman, 'Fresh-water Polyzoa,' figs, i and 2, p. 7, fig. 8, 

 P- 45- 



The animal is figured as it is seen when its lophophore, with the 

 tentacles it supports, the lower segment and outlet of the intestine, 

 and the single nerve ganglion which is situated between them, are 

 retracted into a cavity a, formed for their reception by the invagi- 

 nation of the endocyst into the ' cell/ The double arrow above a 

 indicates that the cavity thus temporarily formed is common to both 

 inhalant and anal orifices. 



b. Nerve ganglion, situated between the two openings of the 



digestive canal, 



c. Lophophore, representing in this figure only one-half of the 



horseshoe-shaped or ' hippocrepian' structure, distinguishing 

 all fresh-water species, except Paludicellea and Urnatellea, 

 and replaced in all marine species, except Rhabdopleura and 

 Pedicellina, by a simple orbicular collar. The hippocrepian 

 form of lophophore may be considered as constituted by the 

 prolongation into two arms of the simple circlet of the 

 marine species. The mouth opens between the two lips of 

 the lophophore in all species, marine and fresh-water alike ; 

 and in all, except Pedicellina, both lips are beset with a row 

 of ciliated tentacles. The two arms of the lophophore are 

 attached to the polypide around its mouth, of which they 

 appear to be a development, and they form thus the roof of 

 its perivisceral space, with which their interior freely com- 

 municates. They are free in the rest of their length, and 

 project from the oral over towards the neural and rectal 

 aspect of the animal. Hence in this diagram the anal 

 orifice occupies the right, and the oral the lelt side of the 

 nerve ganglion. There is no gizzard in fresh-water Polyzoa, 

 but the stomach has a large pyloric coecum ; from the lower 

 end of this a cord is figured as passing to the bottom of the 

 cell. This cord represents the cylindrical granular 'funi- 

 culus;' in connection with which the testis of the Polyzoa, 



