SEtT. IV PHYLITM CCELENTERATA 213 



are, in fact, rows of immense cilia, fused at their proximal ends : 

 their presence and mode of occurrence— arranged in meridional 

 comb-ribs or siuimminc/-plates~a.re strictly characteristic of the 

 class, and indeed give it its name. 



It will be seen at once that— apart from all considerations of 

 internal structure — Hormiphora presents a similar combination ot 

 radial with bilateral symmetry as in some Hydrozoa, such as 

 Ctenaria (Fig. 109, 1), and as in the majority of Actinozoa. The 

 swimming-plates are radially arranged, and mark the eight adradii, 

 but the slit-like mouth and the two tentacles indicate a very 

 marked and characteristic bilateral symmetry. A plane passing 

 through the longitudinal axis of the body, parallel with the long 

 axis of the mouth, is called, as in Actinozoa (see p. 189), the vertical 

 plane : it includes two per-radii, which are respectively dorsal and 

 ventral. A plane at right angles to this, passing through both 

 tentacles, and including right and left per-radii, is called the 

 transverse plane. 



Enteric System. — The mouth leads into a flattened tube (Fig. 

 159, std.), often called the stomach, but more correctly the gullet or 

 stomodceum. It reaches about two-thirds of the way towards the 

 aboral pole, and its walls are produced internally into ridges (std.r.), 

 which increase the area for the absorption of digested food. 

 Living prey is seized by the tentacles, ingested by the aid of the 

 mobile edges of the mouth, and digested in the stomodseum, which 

 is thus physiologically, though not morphologically, a stomach. 

 The products of digestion make their way into the various parts 

 of the canal-system, presently to be described, and indigestible 

 matters are passed out at the mouth. 



Towards its upper or aboral end the stomodseum gradually 

 narrows and opens into a cavity called the infundibuliun (inf.), 

 which probably answers to the stomach of an Actinozoon or a 

 medusa, and is flattened in a direction at right angles to the 

 stomodseum — i.e. in the transverse plane. From the infundibulum 

 three tubes are given off: one, the infandibidar canal (inf. c), passes 

 directly upwards, and immediately beneath the aboral pole divides 

 into four short branches, two of which open on the exterior by 

 minute apertures, the excretory pores (Fig. 160, A, ex. p.). The two 

 other canals given off from the infundibulum are the p)<ii'-rcLdial 

 canals {per. c.) : they pass directly outwards, in the transverse plane, 

 and each divides into two inter-radial canals {int. c), which in their 

 turn divide each into two adradial canals (adr. c.). These succes- 

 sive bifurcations of the canal-system all take place in a horizontal 

 plane (Fig. 160, B), and each of the ultimate branches or adradial 

 canals opens into a meridional canal {mrd. c), which extends up- 

 wards and downwards beneath the corresponding swimming-plate. 

 Furthermore, each per-radial canal gives off a stomodceal canal 

 {std. c), which passes downwards, parallel to and in close contact 



