6 THE CRAG POLYZOA. 



tentacles, which project externally ; and 2, the alimentary canal with its immediate appen- 

 dages, which is lodged in the cavity of the receptacle or " cell " as it is termed. As the 

 tentacles are not contractile, and as, when the body is retracted, they are received completely 

 within the " cell," it follows that the flexible part of the sac which admits of invagination must 

 be at least equal in length to those organs. It is also evident that when retracted the 

 tentacles will be lodged in a tubular sheath formed by the membrane in question, which has 

 thence received the name of " tentacular sheath." The invagination and evagination of 

 this sheath are effected by the action of special muscles, of which a full account will be 

 found in Professor Allman's work above cited ; and the evagination may be perfect as in 

 most of the marine and some few of the fresh-water Polyzoa, or imperfect as in the greater 

 number of the latter class. In which case may be perceived an approach to the formation 

 of an atrium, or common cloacal cavity, such as exists in the Ascidians. The border line 

 between the rigid and flexible portions of the tunic circumscribes an opening, as it were, 

 through which the extrusion and protrusion of the soft parts of the animal take place. 

 This opening, therefore, is termed the " orifice" or " mouth of the cell." In those instances 

 where the cell is of a tubular form the orifice is usually of the same diameter as the tube, but 

 where the cell is of an urceolate or other shape the orifice is usually of less diameter. In 

 a systematic point of view this part of the cell, when the soft parts are removed, affords 

 characters of great importance, as will be afterwards perceived. Other points of im- 

 portance, presented on the exterior of the "cell" are 1, the condition of the surface, 

 whether it be smooth, punctured, dotted, reticulated, &c.; and 2, the existence of certain 

 external organs which will be afterwards more particularly referred to when we come to 

 speak of the Cheilostomatous Sub-order. But one circumstance should be here mentioned. 

 The surface of the cell is in almost all cases in recent Polyzoa covered with a horny 

 cuticle or epidermis, apparently analogous to that on the outer surface of many shells. 

 This epidermis is often so thick as materially to affect the aspect of the surface, by 

 rilling up perforations and smoothing asperities which would otherwise be evident. It is, 

 of course, invariably absent in fossil specimens, and it is therefore in most cases necessary, 

 and in all advisable, when a comparison is instituted between recent and fossil species, to 

 remove the organic matter from the former. Owing to the intractable nature of the 

 covering in question, the only or the most effectual way of doing this is by incineration in 

 the flame of a spirit-lamp, sometimes aided by the blow-pipe. 



Within the " cell" thus formed are contained all the soft parts of the animal. These, 

 however, do not occupy the whole of the space, the remainder being occupied by a " clear 

 fluid, in which float numerous particles of very irregular form and size; and in this fluid 

 may be observed a constant rotatory motion, rendered apparent by the floating corpuscles 

 as they are whirled away under the influence of the currents." The space within -the 

 "cell," thus filled with fluid, is termed the " perigastric" or " perivisceral space," and 

 obviously corresponds with the space occupied by fluid which intervenes between the 

 viscera and the walls of the body in most of the lower animals, whilst the fluid in 



