HYDROZOA 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 unbraiiched 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 



Fio. 187. Group of buds of aPhysophor 

 at the bottom of the pneumatophore. 

 C, Central cavity ; Sk, nectocalyx 

 bud with the ectodcrmal ingrowth. 



FIG. 1 S3. Development of Agalmopls Sarsii (after Metschnikoff). a, Ciliated larva. 5, Stage 

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

 developing pneumatophore (Lf). 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 lacssification. 



