24 MORPHOLOGY. 



4. Orientation. 



The attached extremity of the fixed hydrosoma, or its equivalent iu the free one, is 

 described by Huxley as the proximal end ; that which is placed diametrically opposite to 

 this is the distal end. In the present Monograph I shall employ the terms proximal and distal 

 in the sense thus proposed. Beyond these two aspects none other can be definitely distinguished 

 in the Hydroida. The determination of a right and a left side is impossible, for though an 

 apparent bilateralism occasionally occurs, as, for example, in the planoblasts of Corymorplia (PI. 

 XIX), and in those of Gemmellaria (PI. VII) and Bicorjne (PL VIII) ; these conditions must be 

 regarded rather in the light of an arrest or retardation of the development which in the vast 

 majority of cases results in the symmetrical disposition of the parts radially round a common 

 axis. 



II. Morphology of the Trophosome. 



1 . Ilydrantlis. 



Two distinct portions enter into the composition of the trophosome, namely, the hydranths 

 and the hydrophyton. 



The proper nutritive zooids (woodcut, fig. 2, a, h) which constitute the essential part of the 

 trophosome of the Hydroida have been usually known in common with the zooids of the Actinozoa, 

 or proper coral-animals, by the name of " polypes." It will be more convenient, however, to restrict 

 this term to the Actinozoa, to which Reaumur originally applied it, borrowing the word from 

 Aristotle, who used it, not for coelenterate animals at all, but for the cuttle-fishes ; while for 

 the alimentary zooids of the Hydroida I shall in the present work employ the term litjdranth} 



The hydranth consists essentially of a digestive sac opening at one end by a mouth. Behind 

 the mouth are situated tubular offsets from the sac ; these are known as tentacula. The mouth 

 itself is borne on the summit of a more or less distinctly developed proboscis-like extension of the 

 sac, and to this the name of hypostome may be given. 



The hydranth is developed on some part of the hydrophyton, which usually forms for it a 

 hollow, stem-like support {d), on whose summit or sides it is carried, and with whose cavity its 

 own is directly continuous. 



The form of the hydranth varies. It may be flask-shaped, or fusiform, or nearly cylindrical ; 

 but is in every case more or less mutable, constantly changing its shape as the result of various 

 states of contraction. The hypostome may pass imperceptibly into the body of the hydranth 

 {Hydra, Perigommus), or it may be strongly difierentiated from it, and it then usually presents a 

 trumpet-like form, though frequently acquiring at the same time increased contractility and 



^ Huxley uses in the same sense tlie term polypite, and in former publications of my own I fol- 

 lowed him in the employment of this word. As, however, a virtually identical term (polypide) is in 

 ^ use for the exsertile and retractile portion of a polyzoou, an objection similar to that which renders 

 inexpedient the use of the word polype may be urged against the employment of polypite in the 

 terminology of the Hydroida. 



