io6 ZOOLOGY FOR MEDICAL STUDENTS chap. 



obliterated by specially active outward growth so that the nearly circular 

 outline of the adult is attained. 



Besides Amelia the group Acalephae includes a number of other 

 common and conspicuous Medusae. Cyanea is the common stinging 

 jelly-fish famiUar to bathers. In this case the threadlike tentacles 

 round the margin of the umbrella are very long — it may be several feet 

 in length, and they are richly provided with large and powerful nemato- 

 cysts the discharge of which into the skin produces the stinging sensation. 



The more important of the features which mark off the Acalephae 

 from the Hydromedusae are (i) the greater size and conspicuousness 

 of the Medusa stage of the life-history ; (2) the endodermal position of 

 the gonad ; (3) the presence of gastral filaments ; and (4) the presence 

 in the polyp stage of four longitudinal folds of the endoderm. This 

 last-mentioned feature is of special interest from its foreshadowing a 

 condition seen in a much higher degree of development in the Actinozoa. 



The three types of Coelenterate so far dealt with — the Hydra-like forms 

 (Hydrida) without any medusoid phase in their life history, the Hydro- 

 medusae, and the Acalephae — are included in the first of the two main 

 subdivisions of the phylum Coelenterata, the Hydrozoa. Apart from 

 the frequent occurrence of the Medusa type of structure, the two special 

 distinguishing features of the Hydrozoa are that in the polyp stage the 

 mouth opening is situated at the outer end of the projecting oral cone 

 and the coelenteron is a continuous cavity throughout, although it may 

 be slightly encroached on by inwardly projecting folds of endoderm 

 (scyphistoma). 



The remaining Coelenterates are grouped together as the Actinozoa. 

 Of these we shall take as our first example the genus Alcyonium. 



Alcyonium 



The orange or pale yellow or pale flesh-coloured colonies of this animal, 

 of an irregularly lobed shape which has suggested the popular name 

 " Dead Men's Fingers," are to be found attached to rocks and stones 

 from about low-water mark downwards. A specimen removed from the 

 water (Fig. 45, A) shows no obvious sign of life, but if placed in fresh 

 sea-water there will gradually protrude from its surface numerous semi- 

 transparent polyps showing it to be a colonial organism (Fig. 45, B). 

 The protruding portion of the polyp (Fig. 46) is somewhat cylindrical 



