cii Introduction. 



either of these conditions, may be either dendritic or foliaceous, 

 is more or less flexible or rig-id, accordingly as the ectocyst of 

 each polypide is more or less hardened by calcareous or siliceous 

 deposit. The animals are never aproctous^ but there is only a 

 single orifice in each cell for both mouth and anus, though, when 

 the polypide is protruded, a considerable interval intervenes be- 

 tween these two terminal apertures. The digestive tract is freely 

 suspended in a perivisceral cavity ; which, as these animals possess 

 neither heart nor generative ducts, serves as a receptacle for the 

 blood at all times, and for the products of the generative glands 

 at the periods at which they come to maturity. The mouth opens 

 always between the two lips of a lophophore ; both lips being also 

 always, with the exception of PediceUina, beset with tentacles. 

 The lophophore itself may be circular, as in all marine species, 

 with the exception of the family just named, and of Rliahdopleura ; 

 or it may be prolonged into two arms, extending from the mouth 

 towards the neural or rectal aspect of the animal, as in all fresh- 

 water species, hence called '■ hippocrepian,^ except Paludicella 

 and Uruatella. In the freshwater species the tentacles are con- 

 nected at their bases by an infundibuliform membrane, known as 

 the ' calyx ;' and the mouth is guarded by a valvular organ, the 

 * epistome,^ by virtue of their possession of which they have been 

 classified as ' Phylactolaematous,^ in contradistinction to the ' Gym- 

 nolaematous^ marine sub-orders. The lophophore, being attached 

 round the mouth, either by its entire circumference, as in the 

 marine Polyzoa, or by the base of its two arms, as in the hippo- 

 crepian representatives of the class, forms a roof to the perivisceral 

 space, with which cavity, the cavities of the lophophore, of the 

 entire series of tentacula which it carries, and of the epistome when 

 present, are freely continuous. The tentacles are not flexible in the 

 Polyzoa, with the exception of the Pedicellineae^ and probably some 

 allied forms. Both the external and the internal surfaces of the 

 lophophore and its tentacles are clothed with cilia, the action of 

 which subserves the functions of ingestion of food, and of aeration 

 of the blood. The interior of the perivisceral space is also similarly 

 clothed with cilia ; and the movements of the blood between the 

 mutually intercommunicating cavities of the lophophore with its 

 appendages and the general perivisceral system, are further carried 

 out by the contractions of the muscular fibres of the endocyst, and 



