162 



ZOOLOGY 



SECT. 



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 (J9, s, s r ), some male, 

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

 the bracts or hydrophyllia (hph) 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 ccenosarc, and the various struc- 

 tures attached to it zooids the polypes nutritive zooids, the 



feelers tactile zooids, the 

 sporosacs reproductive 

 zooids, the bracts pro- 

 tective zooids, and the 

 swimming-bells locomo- 

 tory 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 



, . , , . . 



Wnicn the enteric Cavity 



originally extended (Fig. 



FIG. 122. Two stages in the development of Hall- 

 stemma : the endoderm is shaded, the ecto- 



. 



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 



