RESPIRATION AND CIRCULATION. 19 



nutritious elements, portions in tlie form of roundish pellets become separated at intervals 

 from the mass, and are slowly propelled along the tube towards the anus, where, having 

 arrived, they are suddenly ejected into the surrounding water and rapidly whirled away by 

 the tentacular currents. It was these excrementitious pellets that Turpin mistook for 

 unarmed ova in Cristatella. 



(3.) Oiyans of Respiration and Circulation. 



Upon the tentacular crown and the walls of the perigastric space would seem, among the 

 Polyzoa, chiefly to devolve the function of bringing under the influence of the aerating medium 

 the nutritious fluid of their tissues. 



The tentacular crown of a Polyzoon consists of two portions, namely, first, a sort of stage 

 or disc (the lopliophore) (PI. II, fig. 24 ; IX, fig. 7 ; X, figs. 3, 4, U) which surrounds the 

 mouth ; and secondly, a series of tentaatla {!) which are borne upon the margin of the 

 lophophore. Tlie lophophore is throughout almost the entire class of an orbicular figure ; 

 but in the fresh-water genera Cristatella, Lojjhopus, PlumatcUa, and Alcyonella, its neural 

 margin, or that which corresponds to the side of the rectum, is extended into two long 

 triangular lobes or arms, so as to cause the lophophore in these genera to present the form 

 of a deep crescent, round whose entire margin the tentacula are borne in one continuous 

 series. This condition of the lophophore is found in no marine species. In Fredericella the 

 arms of the crescent are obsolete, and the lophophore here may, on a superficial view, 

 appear orbicular; but a careful examination will render manifest its departure from the 

 orbicular form, the side corresponding to the arms of the crescent being slightly prolonged, 

 while the bilaterality is rendered still more decided by the presence of an epistome. 

 Paludicella is the only fresh-water genus, if we except Vrnatellu, not yet sufficiently examined, 

 in which not the slightest trace of bilaterality can be detected (PI. X, fig. 5). With the 

 exception of Pedicellina, which presents a condition of the lophophore altogether unicpie 

 and exceptional,* and some other marine genera in which a slight tendency to assume a 

 bilateral form may be observed, but which never possess an epistome, the lophophore in the 

 marine genera is always orbicular. The lophophore in all the genera forms the roof of the 

 perigastric space ; in the species with crescentic lophophores the arms of the crescent are 

 tubular and open into this space ; the interior of the arms is clothed with vibratile cilia. 



* The structure of PediceU'uia hus been hitherto entirely misunderstood. I have had an oppor- 

 tunity of carefully examining this Polyzoon, and as it is constructed on a plan quite peculiai-, and may 

 tend to throw light on the morphology of other types, it may not be out of place to give a description 

 of it here. 



The general appearance of Pedicellina is that of a gymnolsematous Polyzoon with its nearly 

 spherical cell seated on the extremity of a long peduncle. The gyranolajraatous character, however, is 

 wholly deceptive, and depends on a remarkable condition of the lophophore, which, though at first 

 sight orbicular, is truly bilateral and phylactolrematous, but nevertheless constructed on a type peculiar 

 to itself, and of which no other Polyzoon affords any example, unless further examination shall show a 

 similar disposition in Urnatella. 



The two arms of the lophophore have the tentacula confined to their outer margin. They are 



