of Hijdroids from the White Sea. 225 



This can be explained by the annexed ^,^ 



diagram, showing the genealogy of the types. '•^\" ^ ^ 



As regards the systematic position of j I 



this Hydroid, it undoubtedly forms a dis- ^o^ ' ^oC 

 tinct genus, which I propose to name il/o>jo- 2.5 \_i^ a.« 

 brachium. The species I propose to name I ^.a 



Monohrachium parasitum^ the reason being | 



explained below. If we seek to place it in one _o- 



of the families known to us, it does not pro- ■ * 



perly belong to any of them, in consequence I 



of its unsymmetrical single tentacle, and is 

 therefore analogous to another interesting hydroid, Lar saheU 

 larurn, Gosse. I consider it the best course to form for the above- 

 mentioned hydroid a separate family, Monobrachiidee. Such 

 a new family will find its justification in the great analogy 

 which exists between the two Hydroids, which, therefore, 

 I now proceed to explain. In the first place I take into 

 consideration the analogy of the mode of life of Lar and 

 Monohrachium. They are both met with always on the same 

 species of animals, — Lar on the tube of a worm {Sabella)^ 

 Monohrachium on the Tellina solidida^ and always on the same 

 place, that is, where there are more currents and therefore 

 conveyance of food, upon which the hydroid feeds. Lar is 

 always found at the mouth of a sand-tube, from which pro- 

 ceeds the worm, causing a strong eddy with its gills ; and the 

 Monohrachium on the end of the mollusk (PI, V. fig. 1). This 

 fact is very important, as it indicates a cause for the appearance 

 of these two Hydroids, and consequently it is the cause of the 

 remaining analogies. And actually, if we compare Lar and 

 Monobrachium^'\\kv other Hydroids, it appears thattheyhave the 

 least number of tentacles — Monohrachium one, Lar only two ; 

 therefore it is evident that here the organization is defined. But 

 we cannot regard Lar or Monohrachium as lower forms from 

 which the higher have been developed, because in the Hydroids, 

 almost without exception, we perceive a symmetrical placing of 

 the parts, so that it is evident such unsymmetrical forms as Lar 

 and Monohrachium could not be the stock. After the form like 

 Protohydra Leuchariii^ some form ought to be reckoned having 

 two symmetrically placed tentacles, such as, for instance, 

 Atractylis hitentaculata ( — o — ). Besides this the fully deve- 

 loped medusae of both Hydroids cannot be regarded as of 

 primitive forms ; and every thing shows that as Lar and 

 Monohrachium are products of degradation, they had at first a 

 greater number of tentacles, and that their mode of life, 

 their parasitism, so to speak, on other animals, by which 

 they profit in seeking their food, was the cause that some 



Ann, & Mag. N. Hist. Ser. 4. Vol. xx. 16 



