210 ZOOLOGY SECT, iv 



(mill.} : this end is therefore oral. At the opposite or aboral pole 

 is a slight depression, in which lies a prominent sense-organ (s.o.), 

 to be described hereafter. 



But the most striking and characteristic feature in the external 

 structure of Horrniphora is the presence of eight equidistant meri- 

 dional bands (s.pl.), starting from near the aboral pole, and extend- 

 ing about two-thirds of the distance towards the oral pole. Each 

 band is constituted by a row of transversely arranged comb -like 

 structures, consisting of narrow plates frayed at their outer ends. 

 During life the frayed ends are in constant movement, lashing to 

 and fro, and so propelling the animal through the water. The combs 

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

 their presence and mode of occurrence arranged in meridional 

 comb-ribs, costce, or swimming -plates are 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 of 

 radial with bilateral symmetry as in some Hydrozoa, such as 

 Ctenaria (Fig. 110, 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. An imaginary line passing 

 from the middle of the mouth to the sense-organ is the primary axis. 

 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. 188), the vertical or sagittal 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 or lateral plane. It is along these two 

 planes alone that the body is capable of division into approximately 

 equal halves. 



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

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

 stomodcBum. 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 stomodeeum, 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 stomodeum gradually 

 narrows and opens into a cavity called the infundibulum (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 



