I0 6 ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



H ,-ially active outward growth so that the nearly circular 

 ,,utlin< ih is attained. 



Inrt'lia the group Acalephae includes a number of other 



ornmon and conspicuous Medusae. Cyanea is the common stinging 



irlly-lish familiar to 1 fathers. In this case the threadlike tentacles 



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



!i. and they are richly provided with large and powerful nemato- 



e di>i -har-e of which into the skin produces the stinging sensation. 



The more important of the features which mark off the Acalephae 



irom the Hvdromedusae are (i) the greater size and conspicuousness 



i tin- lite-history; (2) the endodermal position of 



,,id : (:;) the presence of gastral filaments; and (4) the presence 



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



:itioned feature is of special interest from its foreshadowing a 



( oiiditiun >ren in a much higher degree of development in the Actinozoa. 



The three tvpes of ( 'oelenterate so far dealt with the Hydra-like forms 



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



ie, and the Acalephae are included in the first of the two main 



>ul>di\isions of the phylum Coelenterata, the HYDROZOA. Apart from 



irrence of the Medusa type of structure, the two special 



Hurt's 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 nu-lrntrrnn is a continuous cavity throughout, although it may 



litlv encroached on b\ inwardly projecting folds of endoderm 



tiistoma). 



Tin- remaining ( 'oelenterates are grouped together as the ACTINOZOA. 

 () l tli- II take as our first example the genus Alcyonhim. 



AtLCYONIUM 



:>.ilc yellow or pale llesh coloured colonies of this animal, 



il.irlv lobed shape which has suggested the popular name 



" |(l '" 1 ^' ' . ' arc to be found attached to rocks and stones 



<' r mark downwards. A specimen removed from the 



\) shows no obvious sign of life, but if placed in fresh 



1 ill jradnallv protrude from its surface numerous semi- 



- ' 11 i" I"- B colonial organism (Fig. 45, B). 



M of the polyp (Fig. 46) is somewhat cylindrical 



