HYDROIDA 



1) The polyp having only one kind of nematocysts. The claviforin tentacles of simple struc- 

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



a. All tentacles claviform. Fam. Corynida. 



ft. The distal tentacles claviform ; proximally a whorl of filiform tentacles. Fam. Pennariida. 



2) The polyps, besides having oviform or spherical nematocysts, also provided with slenderly 

 oval or nearly cylindrical stinging capsules. 



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



widened distally, but not openly communicating with 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. Myriothelidoe. 



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



cavity. Fam. Milleporidce. 

 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. Tubulariid. 



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

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

 iocerianthida. 



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



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. davida. 



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

 an oral endoderm and a gastric endoderm. 



a. The polyps fusiform with conically pointed proboscis. Fam. Bougainvilliidce. 



ft. The broad body of the polyp well defined from the stem, and provided with a clavi- 

 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 curved. Fam. Stylastcrida. 



II. Athecate Hydroids of the Northern Atlantic. 



Section Capitata Kiihn. 

 Family Corynidae. 



"Hydroids with fusiform or more cylindric polypes, whose oral portion is conically 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 



