13. _Hydrolaridae._--Trophosome (only known in one genus), polyps with two tentacles forming a creeping colony; gonosome, free medusae with four, six or more radial canals, giving off one or more lateral branches which run to the margin of the umbrella, with the stomach produced into four, six or more lobes, upon which the gonads are developed; the mouth with four lips or with a folded margin; the tentacles simple, arranged evenly round the margin of the umbrella. The remarkable hydroid _Lar_ (fig. 11) grows upon the tubes of the worm _Sabella_ and produces a medusa known as _Willia_. Another medusan genus is _Proboscidactyla_. 14. _Monobrachiidae._--The genus _Monobrachium_ is a colony-forming hydroid which grows upon the shells of bivalve molluscs, each polyp having but a single tentacle. It buds medusae, which, however, are as yet only known in an immature condition (C. Mereschkowsky [41]). 15. _Ceratellidae._--Trophosome polyps forming branching colonies of which the stem and main branches are thick and composed of a network of anastomosing coenosarcal tubes covered by a common ectoderm and supported by a thick chitinous perisarc; hydranths similar to those of _Coryne_; gonosome, sessile gonophores. _Ceratella_, an exotic genus from the coast of East Africa, New South Wales and Japan. The genera _Dehitella_ Gray and _Dendrocoryne_ Inaba should perhaps be referred to this family; the last-named is regarded by S. Goto [16] as the type of a distinct family, _Dendrocorynidae_. Doubtful families, or forms difficult to classify, are: Pteronemidae, Medusae of Cladonemid type, with hydroids for the most part unknown. The British genus _Gemmaria_, however, is budded from a hydroid referable to the family _Corynidae_. _Pteronema_ (fig. 53). _Nemopsidae_, for the floating polyp _Nemopsis_, very similar to _Tubularia_ in character; the medusa, on the other hand, is very similar to _Hippocrene_ (_Margelidae_). See C. Chun (HYDROZOA [1]). _Pelagohydridae_, for the floating polyp _Pelagohydra_, Dendy, from New Zealand. The animal is a solitary polyp bearing a great number of medusa-buds. The body, representing the hydranth of an ordinary hydroid, has the aboral portion modified into a float, from which hangs down a proboscis bearing the mouth. The float is covered with long tentacles and bears the medusa-buds. The proboscis bears at its extremity a circlet of smaller oral tentacles. Thus the affinities of the hydranth are clearly, as Dendy points out, with a form such as _Corymorpha_, which also is not fixed but only rooted in the mud. The medusae, on the other hand, have the tentacles in four tufts of (in the buds) five each, and thus resemble the medusae of the family _Margelidae_. See A. Dendy [12].