238 Description of the Plates. 



and also in the fresh -water species, certain non-sexual repro- 

 ductive gemmae are developed. The ovary is figured on the 

 oesophageal side of the cell, and towards its upper part. 

 From the bottom of the cell, where in most cases a distinct 

 septum separates the several polypides, retractor muscles are 

 figured as passing upwards and attaching themselves to the 

 sides and upper part of the oesophagus. As the nerve gan- 

 glion has been described as sending filaments to these 

 muscles, as also to the evaginable endocyst, lophophore, 

 tentacles, epistome, and digestive tract, it would appear to 

 correspond not only to the cerebroid ganglia of higher 

 molluscs, but also to the parieto-splanchnic. A nerve collar 

 has also been figured as passing round the oesophagus from 

 the infra-oesophageal mass. The Polyzoa are devoid of a 

 heart; and the blood contained in their perigastric cavities 

 is aerated and kept in motion partly by the ciliary action of 

 the inner surface of the endocyst; partly by its muscular 

 contractions; and partly by the muscular movements re- 

 tracting and protracting the entire polypide; which canse 

 the contents of the perivisceral and of the tentacular cavi- 

 ties to be freely interchanged. 



For a figure giving full details on a large scale of the lophophore, 

 and its relations to the mouth, anus, nerve-ganglion, and 

 epistome, see AUman^s Monograph, ' Fresh- water Polyzoa,' 

 published by the Ray Society, pi. ii., fig. 24; and passion for 

 information as to the entire class, and especially as to the 

 fresh-water representatives of it. 



For an enlarged view of the nerve-ganglion, drawn as surrounding 

 the oesophagus with a collar, see Nitsche, Dubois Reymond, 

 and Reichert, Archiv., 1868, Taf. xiii., fig. 23. See also Du- 

 mortier and Van Beneden, Mem. Acad. BruxelL, torn, xv., 

 pi. iv., fig. 5, p. 85 ; Hyatt, Proceedings of Essex Institute, 

 Salem, U. S. A., v. 4, Oct., 1867, p. 107. 



For a figure of a marine species, showing its orbicular lophophore, 

 ovicell, and avicularia, see Bronn, Klassen und Ordnungen des 

 Thierreichs, iii. 9, Taf. v., fig. 3. 



For the existence of a nerve-system common to an entire colony, 

 see Fritz Muller, Archiv. fur Naturgeschichto, i860, p. 311. 



