Introduction to Animal Morphology. 81 



The group is divisible into two classes : 



CLASS i. HYDROZOA Saccular or tubular personae, 

 either single or in varying degrees of aggregation, 

 with no separate digestive sac, and with the repro- 

 ductive organs external. To each individual, whether 

 persona or colony, the name Hydrosoma is given. 

 The following sub-classes are here included : 



Sub-class i . Hydroida locomotion never by meri- 

 dional rows of cilia ; generative elements discharging 

 themselves externally. The zooids serving for the 

 nutrition of the hydrosoma are never joined in free 

 swimming colonies with the reproductive zooids. 



Four orders are included : 



i . Eleutheroblastea (A llman}. Hydras are separate 

 personae attached at will by an aboral disc, and con- 

 sisting of a tubular digestive cavity with no anus. The 

 4-10* tubular tentacles are prolongations of both endo- 

 and ecto-derm (Fig. 10 T), and vary in 

 length from \" to 10 times the length 

 of the body i which is -oS" - '4", or even 

 more). The ectoderm consists of large 

 nucleated cells, from whose bases fila- 

 mentary processes are continued in- 

 wards ;Ficj. ii, A. Klti/iciibcrgrG- 



1s these as nerve cells, and the pro- 

 S muscular, thus forming the 

 simplest differentiated neuro-muscular .',' 



n these cells are j,'; '*^ 



.ller irregular interstitial cells not tunning a 



ial lavr, but surrounding the bases of tin- first 



.mo! containing the nettle ci-lls, which often 



sunk in the sides of the lar-vr <( todennal cells. 



K 



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