226 M. C. Mereschkowsky on new S2)ecies 



of the tentacles, from not being required, were thrown off. 

 Actually the strong eddies formed by the worm as also by the 

 siphons and the foot of the Tellina, infallibly bring portions of 

 food, which falling into the mouth of the hydroid do away 

 with that necessity for a great number of tentacles which 

 existed when they led a different kind of life ; and thus there 

 appears another analogy between the two hydroids — a more 

 complex form of medusa in comparison with a trophosome 

 reduced to a more retrograde form. And it is clear that the 

 medusa when parted from the trosophome loses those conditions 

 in which the trophosome continues to grow : it is subject to 

 new conditions nearly like those of other medusee ; it swims 

 freely on the surface ; and for the struggle of life it requires a 

 good organization. The third analogy is the asymmetry 

 clearly indicated in both the hydroids. The type of the 

 m.edusag may serve to show the condition of the trophosome 

 before its degradation. As regards Lar^ it might be thought 

 that it proceeded from a hydroid which had at first six tenta- 

 cles ; and this supposition reposes on the fact that the medusa 

 has six radial canals and six tentacles, as also on the dis- 

 position of its only pair of tentacles, which are not symme- 

 trically placed one opposite the other, but nearly at an angle 

 of 60°, exactly as it would be if from a sexradial type four were 

 taken away (see figure). In this manner the family 

 Laridse forms a separate branch from the type 2x3. „.';w'''_ 

 Analogically to this I think it follows that, after all /Q^ 

 that has been said, MonohracMuin parasitum forms a 

 separate family, Monobrachiidte, as a branch detached from the 

 type 2x2, as its medusa has only four radiating 

 canals (see figure). The genealogical tree opposite • 



may elucidate the,p,lace of Monohrachium among the "9 



other types. f 



This Hydroid may be briefly characterized as follows : — . 



Order HYDEOIDA. 

 Suborder Athecata. 



Fam. Monobrachiidae. 



Hydrocaulus not developed; hydranth with a single filiform 

 tentacle ; central mouth without lobes. Gonophores medusi- 

 form planoblasts with four radiating canals. 



MONOBRACHIUM, n. g. 



Hydrorhiza consisting of a continuous expansion, not com- 

 posed of a mass of anastomosing stolonic tubes ; hydranth 



