RESPIRATION AND CIRCULATION. 21 



structure ; it resists putrefaction longer than the external cellular layer, and forms the im- 

 mediate walls of the tubular cavity. Nervous filaments, and certain bands, probably mus- 

 cular, to be presently described, may be traced as far as the root of each tentacle, and 

 doubtless also enter into its structure. In Cristatella, a minute cavity, which looks as if it 

 were cut off from the rest of the tube, may be very plainly seen in the extremity of each 

 tentacle ; this condition would also seem to exist in other genera, but it is nowhere so well 

 marked as in Cristatella. 



In all the fresh-water genera, with the exception of Paludicella, and possibly of 

 Urnatella, the entire plume of tentacula is surrounded at its base by an exceedingly delicate 



water Phylactolffimata is probably united. In order to understand this relation, it is necessary to 

 conceive of the polypide as partially retracted ; the invaginated portion of the endocyst must then be 

 viewed as adherent to the external portion externally, and to the tentacula (and calyx ?) internally. 



From this arrangement it is obvious, that the exsertion and retraction of the polypide must be 

 very limited. "When the polypide desires to withdraw under cover of the cell, the free portion of 

 each tentacle is rolled inwards as far as the margin of the calyx-like cup, and then the mouth of the 

 cup is closed over the whole by the action of a well-developed sphincter muscle, the tentacular crown 

 being at the same time slightly drawn backwards by some retractor fibres, which may be seen extend- 

 ing from the upper part of the pharynx to the base of the cell. The sphincter would seem to represent 

 a Condensed band of the parietal muscles of other Polyzoa, or it is probably homologous with the 

 vaginal sphincter, which is nothing more than a peculiar development of these muscles in -the invagi- 

 nated endocyst. The retractor fibres are obviously homologous with the great retractor muscles 

 of the others. 



The mouth, which occupies its normal position in the body of the lophophore, opens into an 

 (Esophagus, which after a short course terminates in the stomach. This is a large sac which lies in 

 the bottom of the cell; close to the pyloric orifice it becomes much constricted, and this constricted 

 portion passes into the intestine whose origin is nearly on a level with the cardiac orifice, but separated 

 from it by a considerable space. The intestine passes first forwards and tovpards the neural side, then 

 turns towards the mouth to open after a wide dilation between the arms of the lophophore in the bottom 

 of the tentacular crater. 



The concave margin of the body of the lophophore is raised into a kind of thickened rim, which 

 arches over the mouth, so as to constitute an epistome thickly ciliated on its oral aspect, but not pro- 

 vided with muscles like the epistome of the ordinary phylactolpematous Polyzoa. 



The hepatic layer, which, in other Polyzoa, forms a continuous lining extending over nearly 

 the whole of the internal surface of the stomach, is here confined to the anterior portion of this sac, in 

 the space between the cardiac and pyloric orifices, where it constitutes a dark brown glandular mass 

 composed of distinct cells, which are filled with the characteristic brown secretion. The rest of the 

 stomach, as well as the oesophagus, is lined with vibratile cilia, which are especially developed in the 

 vicinity of the pylorus. 



The ovary is an irregularly shaped mass, situated between the cardiac and pyloric orifices of the 

 stomach, and the animal appears to be unisexual, the testis occupying the place of the ovary in other 

 individuals. 



The stalk on which the cell containing the polypide is elevated, constitutes a very characteristic 

 feature of PedkeUina. It is a tube in whose walls both ectocyst and endocyst can be demonstrated ; 

 it contains straight muscular fibres, which extend from the base of the cell to the point of attachment 

 of the stalk ; and besides these, more delicate circular fibres can also be detected in it. By tlie action 

 of the straight and circular fibres, various motions, especially those of flexion and extension, can be 



