152 J. HOPKTNSON ON BraTISH GEAPT0LITE8. 



§ I. Structure. 



Assuming for the present that the Graptolitid£e form a distinct 

 order of the class Hydrozoa, it seems necessary to explain the 

 terms proposed for this class hy Dr. Allman and Professor 

 Huxley, as, though now generally adopted, they are by no means 

 ■widely known. 



The body of every Hydrozoon consists of a sac composed of two 

 membranes, an outer, called the ectoderm, and an inner, the endo- 

 derm. This entire double-walled body is termed a hydrosome, and 

 consists in its simplest form, as seen in the fresh water Hydra, of a 

 disc of attachment, the hydrorhiza; a sac for the digestion of 

 food, the polypite ; and processes for seizing food, the tentacles. 

 At certain seasons reproductive organs are developed. 



In the majority of the Hydrozoa there are several polypites 

 organically united by a common connecting fleshy basis, termed the 

 ccBnosarc, forming, in the Corynidae and Sertulariad^e, the nearest 

 allies of the Graptolitid®, a branching stem, a transverse section 

 of which is usually nearly circular. 



The growth of every hydrosome takes place from one end, which 

 does not further develope. This end may be in any position, up- 

 wards, downwards, or sideways. It is termed the proximal end ; the 

 other, or growing extremity, being termed the distal end. The 

 proximal end forms a hydrorhiza, either expanding into a disc, or 

 sending out radicles, by which it attaches itself to other bodies. 



That portion of the hydrosome which intervenes between the 

 hydrorhiza and the first polypite is termed the hydrocaulus. The 

 opposite ends of the individual polypites are also termed proximal 

 and distal; the proximal being attached to the supporting ceenosarc, 

 and the distal being armed with filiform tentacles. 



The individual polypites of many Hydrozoa are protected by 

 cup-like receptacles, commonly called polype cells, entirely composed 

 of the cuticular layer of the ectoderm. These are termed hydro- 

 thecoB.* The ctenosarc also has a protecting covering, tei-med the 

 periderm ; the whole covering of the soft parts being called the 

 polypary. The tentacles are never protected by a special covering. 



* It may be objected that the word cell or cellule is applicable, and is more 

 convenient than hydrotheca ; but to a microscopist a cell is the ultimate particle 

 of Uvinf? tissue, and a cellule is but a little cell. 



The word mouth has likewise been used for the aperture or orifice of each 

 hydrotheca ; but the opening through which the polypite protudes its tentacles, 

 as well as the distal extremity of its body, cannot certainly be called a mouth ! 



