276 



NATURAL HIS TOST. 



appendicular organs as observed by him in living specimens, off the coast of Tierra del Fuego in 

 1846. 



The subjoined table gives the classification of the Sea-mosses, and the approximate inter- 

 relationships of the minor groups, commencing with the higher forms. Sub-orders and genera known 

 to be represented by fossil species are printed in italics : 



CheiZostomata, marine, Flustra, Bugula, Cellepora, Membranipora. 



Cyclostomata, marine, Stomatopora, Hornera, Fenestella, Lichenopora, 



Ctenostomata, marine, Bowerbankia, Victorella, Mimosella. 



Paludicella, fresh- water, Paludicella, only. 



ECTOPROCTA 



ENDOPROCTA 



Lophophea, fresh- water, Lophopus, Alcyonella, Plumatella, etc. 



PTEROBRANCHIATA. 

 Pedicellina. Podostomata (mouth- footed). 



iPedicellina, marine. Rhabdopleura, marine, only.* 



Urnatella, fresh-water. Cephalodiscus dodecalophus. 

 Loxosoma, marine. 



ORDER HOLOBRANCHIATA. 

 THE ECTOPROCTA. THE CHEILOSTOMATA. 



Members of this highly-organised group are usually, as their name implies, distinguished by the 

 presence of a horny lip or shutter, which, moved by special muscles, effectually closes the orifice of the 

 cell on the withdrawal of its animal occupant. The external layer of the cell wall (ectocyst), 

 sometimes fleshy or horny, is more often of a calcareous nature. The Cheilostomata are further 

 characterised by the frequent presence of those singular outgrowths of the colonial skeleton the 

 vibracular and avicularian appendages already described. The former occur more 

 frequently than the latter, with which, moreover, they are occasionally associated. 

 The bases of these organs often pi-eserved in a fossil state aid in the determina- 

 tion of genera. The marsupium, or " brood-chamber," also distinguishes these Moss- 

 animals, which undoubtedly exemplify the most perfect form of bryozoonal organisa- 

 tion, although, at the same time, that which farthest removed from the molluscan 

 type illustrates the effects of degeneration resulting from the adoption of a 

 stationary life. In Moss-animals with a circular-mouthed cell (Cyclostomata, Fig. 27), 

 the ectocyst is likewise more or less encrusted with a calcareous secretion, and 



f- 27 STOMA- the OI> ifi ce f the usually long and tubular cell is generally unprotected by any 



TOPORA DICHO- apparatus for its closure and protection of the more simply-organised animal. But 

 GREAT* OOLITE '^ * mem b ers of this group occasionally possess a modified operculum, which links them 

 with the perfectly " lip-celled " Moss-animals, some of which in turn present external 

 features of the Ctenostomata f (Fig. 28), a third sub-order, which includes a number of remarkable 

 species, for some are fleshy, irregular, and spongious ; others horny, plant-like, with flexible movable 

 cells. The ectocyst, never calcareous, is either of a horny-membranous or membrane-gelatinous 

 nature, and in many cases the cells arise at intervals from a creeping tubular stem, the stolon 

 (Fig. 28, s). A portion of the tentacular sheath is fringed with a row of bristles connected by a thin 

 membrane, which, drawn together on the retreat of the polypide, closes in and forms a hairy 

 operculum or " comb-like " protection above the tentacles when those organs are retracted within the 

 lower and harder portion of the cell. Some species inhabit brackish water, and exhibit modi- 

 fications of the tentacular crown and other resemblances to the fresh- water forms. 



The sub-order Paludicella, is at present represented by a single, branched, fresh-water Moss- 

 animal, which, of timid habit and a lover of obscurity, dwells in slowly-running waters, whence its 

 generic name is derived. The animal of Paludicella inhabits a club-shaped, divided cell. The loop 

 is circular, and, like all the foregoing members of the Gymnolsematous section, the gullet is 

 unprotected by the "epistome." All the remaining Ectoproctous forms are comprised within the 

 sub-order Lophophea, founded on the genus Lophopus the famous " Bell-flower " animal first 

 * A second supposed fresh- water Pterobranehiate has been described by Mr. Hincks, from Canada, t Or., kteis, a comb. 



