GKOUPS OF POLYZOA 909 



on account of the remarkable distinctness with which the various 

 parts of their organisation may be seen and the very beautiful man- 

 ner in which their ciliated tentacula are arranged upon a deeply 

 crescentic or horseshoe-shaped lophophore. By this peculiarity the 

 fresh-water Polyzoa are distinguished from the marine ; and they, 

 with the marine Rhabdopleura, may be further distinguished by the 

 possession of an epistome, or moveable process above the mouth, 

 whence Professor Allman calls them the Pkylactofamata, as com- 

 pared with the others, which are Gymnolcemata, or have no epistome. 

 The cells of the Phylactolcemata are for the most part lodged in a 

 sort of gelatinous substratum which spreads over the leaves of 

 aquatic plants, sometimes forming masses of considerable size ; but 

 in the very curious and beautiful Cristatella the polyzoarv is un- 

 attached, so as to be capable of moving freely through the water. 1 



In the marine Polyzoa, constituting by far the most numerous 

 division of the class, the anus opens either outside (Ectoprocta) or 

 within (Entoprocta) the circlet of tentacles ; the former comprise 

 three groups : I. Cheilostomata, in which the mouth of the cell is 

 sub-terminal, or not quite at its extremity (fig. 688). is somewhat 

 crescentic in form, and is furnished with a movable (generally mem- 

 branous) lip. which closes it when the animal retreats. This includes 

 a large part of the species that most abound on our own coast, not- 

 withstanding their wide differences in form and habit. Thus the 

 polyzoaries of some (as Flustra) are horny and flexible, whilst those 

 of others (as Eschara and Retepora) are so penetrated with calcareous 

 matter as to be quite rigid ; some grow as independent plant -like 

 structures (as Bugula and Gemellaria), whilst others, having a like 

 arborescent form, creep over the surfaces of rocks or stones (as 

 Hippothoa) ; and others, again, have their cells in close apposition, 

 and form crusts which possess no definite figure (as is the case with 

 Lepralia and Membranipora). II. The second order, Cydostomata, 

 consists of those Polyzoa which have the mouth at the termination of 

 tubular calcareous cells, without any movable appendage or lip (fig. 

 689). This includes a comparatively small number of genera, of which 

 Crisia and Tubulipora contain the largest proportion of the species 

 that occur on our own coasts. III. The distinguishing character of 

 the third order, Ctenostomata, is derived from the presence of a comb- 

 like circular fringe of bristles, connected by a delicate membrane, 

 around the mouth of the cell, when the animal is projected from it, 

 this fringe being drawn in when the animal is retracted. The poly- 

 zoaries of this group are very various in character, the cells being 

 sometimes horny and separate (as in Farrella and Boiverbankia), 

 sometimes fleshy and coalescent (as in Alcyonidium). IV. In the 

 Entoprocta, which are represented by Loxosoma and Pedicellina, 

 and are doubtless the most archaic of the true Polyzoa, the lopho- 

 phore is produced upwards on the back of the tentacles, uniting 

 them at their base in a sort of muscular calyx, and giving to the 

 animal when expanded somewhat the form of an inverted bell, like 



