754 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Even in infested ground certain plants remain free. Slight differences in 

 thickness of cell-wall, &c., mean much to the parasite. A detailed tabular 

 survey of its relation to numerous plants is given. The persistence of 

 Tylenchus for generations on one kind of plant may to a certain extent unfit 

 it to attack another ; the difference resulting is rather physiological than 

 morphological. Just as Bacteria morphologically the same are often very 

 different physiologically, so vpith Tylenchi ; and just as a Bacterium may 

 have its virulence attenuated by a given culture, so T. devastatrix, as far as 

 hyacinths are concerned, may be said to be attenuated by culture for several 

 generations on rye. 



Although the TylencM are true plant parasites, it follows from the 

 nature of most of the plants which they infest that they spend part of 

 their life in the ground. It is different, however, with those infesting 

 hyacinths and bulbous stems. In spring these usually migrate from bulb 

 to leaves, retiring again to the bulb as the leaves die off. They pass from 

 old bulb to young bulb, and thus never enter the ground. The "rye- 

 worms " pass from the soil into the young plants, remain there and multiply 

 till the grain ripens and the stem and leaves begin to wither. Then they 

 retire to the soil again. The life-history varies with the plant infested. 

 They may remain a long time, over a year sometimes, latent in the ground, 

 probably in a lethargic state, but this can only occur in the upper drier 

 layers of the soil, 



y, Platyhelmintlies, 



Relation of the Nemertea to the Vertebrata.* — Prof. A. A. W. 



Hubrecht devotes bis memoir on this subject to the establishment of the 

 following proposition : — " More than any other class of invertebrate animals 

 the Nemertea have preserved in their organization traces of such features 

 as must have been characteristic of those animal forms by which a transition 

 has been gradually brought about from the archicoelous Diploblastic 

 (Coelenterate) type to those enterocoelous Triploblastica that have after- 

 wards developed into the Chordata (Urochorda, Hemichorda, Cephalo- 

 chorda, and Vertebrata). 



In support of this proposition it is pointed out that the Nemertea 

 present the following Coelenterate characteristics : the presence of nemato' 

 cysts in the proboscidian epithelium, the elaborate nerve^plexus in the 

 integument and its histological features, the presence of epiblastic muscle- 

 fibres separate from the general body-musculature, the presence and the 

 chemical constitution of a sometimes very massive intermuscular jelly by 

 which the other organs are at the same time surrounded, the mode of 

 development of the mesoblast (at least in Lineus obscurus), which is less 

 specialized than in most other Invertebrates, and lastly, the absence of any 

 distinct enteroccele. The following, on the other hand, are the points of 

 resemblance to the Chordata ; the general features of the nervous system, 

 the presence of a homologue of the hypophysis cerebri as a massive and 

 important organ (the proboscis), the presence of tissues which may have 

 become converted into the notochord (viz. the material of which the pro- 

 boscidian sheath is built up), and the respiratory significance of the anterior 

 portion of the alimentary tract. 



Prof. Hubrecht's speculations are based on the conviction that new 

 combinations or organs do not appear by the action of natural selection 

 unless others have preceded, from which they are gradually derived by a 

 glow change and differentiation. 



* Quart. Journ. Micr. Sci., xx«Fii. (1887) pp. 605-44 (1 pi). 



