HTDROZOA SIPHONOPHOBA . 



245 



possess a circle of tentacles. They always, however, have a long 

 tentacle arising from their base. This tentacle can be extended to a 

 considerable length, and be retracted 

 into a spiral coil. It rarely has a 

 simple form, but, as a rule, it bears 

 a number of unbranched lateral 

 twigs, which are also very contrac- 

 tile. These tentacles are invariably 

 beset with a great number of nema- 

 tocysts, which in many places are 

 closely packed and have a regular 

 arrangement. These aggregations 

 of thread-cells are especially found 

 on the lateral branches of the 



tentacles, and give rise to large, brightly-coloured swellings, the 

 batteries of nematocysts. The batteries show considerable variations 



a I 



FIG. 187. Group of buds of a Physophor 

 at the bottom of the pneumatophore. 

 C, Central cavity ; S/c, nectocalyx 

 bud with the ectodermal ingrowth. 



FIG. 188. Development of Agnl mojiix Siirxil (after Metschnikoff). a, Ciliated larva, b, Stage 

 with developing hydrophyllium (D). c, Stage with cap-shaped hydrophyllium (D) and 

 developing pneumatophore (/). d, Stage with three hydrophyllia, (D, D', D"), polyp 

 (P), and tentacle. 



in form in the various species, genera, and families, and such varia- 

 tions afford valuable characters for systematic classification. 



