134 COMPAEATIVE ANATOMY. 



lives. lu Filigraua tlie stalk of the opei'culuin retains some of its 

 primitive characters by being feathered. But this feathered arrange- 

 ment may be lost (Serpula), and then the operculum, during its 

 development, passes through stages which are permanent in other 

 forms. A calcified layer is often secreted in this apparatus, which 

 owes its origin to adaptation ; it covers the free flattened end like a 

 disc. In some cases the widened opercular stalk takes up the ova, 

 and functions as a brooding pouch (Spirorbis spirillum). Thus we 

 find one and the same organ passing through a series of the most 

 varied relations, far removed from its original significance, and 

 caused by certain external relations. In addition to the feelers 

 there are special tentacles in the Chaetopoda, which are shorter, 

 but contractile (Fig. 55, a). 



The tentacles of the Bryozoa are structures of this kind ; they 

 have the form of filamentous ciliated and contractile processes of a 

 discoid or lobate extension of the oral end of the integument (lopho- 

 phore). The discoid form of lophophore, in which the mouth is 

 placed in the centre, is the most common. In the other case, the 

 lophophore is drawn out into two processes, so as to have a horse- 

 shoe shape (Fig. 60, B br). 



In Pedicellina and Loxosoma, the tentacles, which beset the edge 

 of a discoid surface, which carries both mouth and anus, are simpler 

 in character ; they are not hollow internally, like the tentacles of the 

 other Bryozoa. 



§ 105. 



Another group of appendages is represented by the locomotor 

 processes developed in Chastopoda, which are lateral processes of 

 the metameres of the body, the foot-stumps or parapodia (Figs. 

 55, 56, p). They are always arranged in pairs, of which there may be 

 one or two on each segment. When there are two, one pair occupies 

 the dorsal, and the other the ventral portion of the side of the body. 

 They carry seise, and often also filamentous appendages (cirri), which 

 vary greatly in form, and may be larger than the parapodia, or may 

 even take the place of these appendages, when the latter are atrophied. 

 The dorsal and ventral appendages of either side are sometimes 

 closely approximated; there are all kinds of intermediate steps 

 between this stage, and that in which they are completely fused 

 (Syllida3). Such a fused appendage occupies the side of the body, and 

 carries the secondary appendages (setffi and cirri), which, in others, 

 are distributed to the dorsal and ventral parapodia. The cirri 

 appear to be atrophied in the Tubicolse, where they cannot have any 

 physiological significance, owing to the body occupying a tube, 

 which has sometimes the form of a shell. 



The parapodia are developed in very various degrees, and are com- 

 plicated by their relation to groups of setae. A metamorphosis is 

 effected by a widening of the ends of the separate, or of fused 

 parapodia, or rather of their cirri, to form swimming-plates (Phyllo- 

 docidee). The elytra are special appendages of the parapodia 



