HYDROIDA 1 1 



i) The polyp having only one kind of neniatocysts. The claviform tentacles of simple struc- 

 ture, having no central cavity and no particularly developed supporting lamella. 

 a. All tentacles claviform. Fam. Corynidcr. 



[i. The distal tentacles claviform; proximally a whorl of filiform tentacles. I'-am. PoDiariidcr. 

 2) The polyps, besides having oviform or spherical neniatocysts, also provided with slendcrl)- 

 oval or nearly cylindrical stinging capsules. 



a. Without calcareous skeleton. The claviform tentacles having a central cavity, greatly 

 widened distally, but not openly communicating witli the gastric cavity of the polyp. 

 The supporting lamella, in the outer portion of the tentacles, developed into a thick 

 radial fibrous supporting tissue. Fam. Myriothclida\ 

 p. Colonies with calcareous skeleton. The tentacles of simple structure, having no central 

 cavity. Fam. Millcporidcc. 

 b. All the tentacles of the polyp filiform, arranged in two main circles: 



1) The proximal (basal) whorl of tentacles leaning against a well-developed mesogloual cushion. 

 The polyps of radial symmetrical structure. Fam. Tnbulariidcc. 



2) The mesoglaal formation at the basis of the proximal whorl of tentacles almost wanting, 

 owing to radial canals. The structure of the polyps bilaterally symmetric. Fam. Braiich- 

 ioccrianthidce. 



II. The neniatocysts always only .slender, the small ones quite rod-shaped. (Sectio Filifera Kiilin). 



a. Colonies without calcareous skeleton. 



1) The tentacles irregularly spread all over the polyp, or reducible to a single large tentacle. 

 The endoderm not differentiated into oral endoderm and gastric endoderm. Fam. Clavidce. 



2) The tentacles forming a main circle round the polyp. The endoderm differentiated into 

 an oral endoderm and a gastric endoderni. 



a. The polyps fusiform with conically pointed proboscis. Fam. Bougainvilluda;. 



j3. The broad body of the polyp well defined from the stem, and provided with a cla\i- 

 form proboscis placed with a narrow basis on the whorl of the tentacles. Often two 

 kinds of nematocysts, large ones and small ones. Fam. Eudendriida:. 



b. Colonies with calcareous skeleton and with two kinds of nematocysts, the large ones being 

 slenderly oval or nearly cylindric, frequently slightly cur\-ed. Fam. Stylastcridcc. 



II. Athecate Hydroids of the Northern Atlantic. 



Section Capitata Kiihn. 

 Family Corynidae. 



"Hydroids with fusiform or more cylindric pohj)es, whose oral portion is coiiicall\- pointed. 

 The stinging capsules are large, oviform, or almost globular. All the tentacles of the polype are 

 capitate with the stinging cells mainly concentrated on the thickened distal portion. The structure 



2* 



