Iritrodudion to Animal Morphology. 8i 



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 

 themseWes 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 [Allmaii). — 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(v/hich is -08"- •4''', or even 

 more). The ectoderm consists of large 

 nucleated cells, from whose bases fila- 

 mentary processes are continued in- 

 wards (Fig. II, A). Kleincnberg re- 

 gards these as nerve cells, and the pro- 

 cesses as muscular, thus forming the 

 simplest differentiated neuro-muscular ^]tl^tlc\^I'^l'^lyixy 

 apparatus. Between these cells are ^-^^p^-'genous pa- 

 smaller irregular interstitial cells, not forming a 

 special layer, but surrounding the bases of the first 

 series, and containing the nettle cells, which often 

 appear sunk in the sides of the larger ectodermal cells. 



* Rarely twelve or more. 

 G 



