SECT, iv PHYLUM CCELENTERATA 161 



through the water. Below the last nectocalyx the character of the 

 structures borne by the stem changes completely: they are of 

 several kinds, and are arranged in groups which follow one 

 another at regular intervals, and thus divide the stem into seg- 

 ments, like the nodes and internodes of a plant. 



Springing from certain of the " nodes "are unmistakable polypes 

 (p), differing however from those we have hitherto met with in 

 having no circlet of tentacles round the mouth, but a single long 

 branched tentacle (t) arising from the proximal end, and bearing 

 numerous groups or " batteries " of stinging-capsules (ntc). In 

 the remaining nodes the place of the polypes is taken by dactylo- 

 zooids or feelers (dz) mouthless polypes, each with an unbranched 

 tentacle springing from its base. Near the bases of the polypes 

 and dactylozooids spring groups of sporosacs (B, s, /), some male, 

 others female ; and finally delicate, leaf-like, transparent bodies 

 the toads or kydrophyllia (hpk) spring from the " internodes " and 

 partly cover the sporosacs. 



It is obvious that on the analogy of such a hydroid polype as 

 Obelia, Halistemma is to be looked upon as a polymorphic floating 

 colony, the stem representing a coenosarc, and the various struc- 

 tures attached to it zooids the polypes nutritive zooids, the 

 feelers tactile zooids, the sporosacs reproductive zooids, the bracts 

 protective zooids, and the swimming-bells tocomotory zooids. The 

 float may be looked upon as the dilated end of the stem, which 

 has become invaginated or turned-in so as to form a bladder 

 filled with air, its outer and inner surfaces being furnished by 

 ectoderm, and the middle portion of its wall by two layers of 

 endoderm, between which the enteric cavity originally extended 

 (Fig. 120, pn). The upper or float-bearing end is proximal 

 i.e. answers to the attached end of an Obelia-stem : it is the 

 opposite or distal end which grows and forms new zooids by 

 budding. 



In some Siphonophora the bracts contain indications of radial 

 canals, so that these structures, as well as the swimming-bells 

 and sporosacs, are formed on the medusa-type, while the hydranths 

 and feelers are constructed on the polype-type. 



It will be noticed that the radial symmetry, so characteristic 

 of most of the Hydrozoa previously studied, gives way, in the 

 case of Halistemma, to a bilateral symmetry. The swimming-bells 

 are placed obliquely, and the mouth of the bell is not at right 

 angles to the long axis, so that only one plane can be taken 

 dividing these structures into two equal halves : the same applies 

 to the polype and feelers with their single basal tentacle. When 

 first formed the various zooids are all on one side of the stem, but 

 the latter becomes spirally twisted during growth, and so causes 

 them to arise irregularly. 



VOL. I M 



