Little by little, Ludmilla Metchnikoff's health improved and her husband was able to resume work. He studied aquatic animals in view of the genealogy of inferior groups, and, amongst others, studied the Tornaria, which was believed to be the larva of the star-fish. However, to his astonishment, he ascertained that, in spite of great similarity, it was not the larva of an Echinoderm, but that of one of the Balanoglossi, of the worm type. This fact established a link between the Echinodermata and worms, a very important result from the point of view of the continuity of animal types.
Metchnikoff felt his courage returning and also his natural high spirits. His wife, who was a clever draughtswoman, helped him with the drawings for his memoir, and both felt happy and contented; this stay at Spezzia was a real oasis in their life.
When the heat became excessive they went to Reichenhall, a summer resort prescribed by the doctor. There, Metchnikoff completed his previous researches on the development of the scorpion, and finally established the fact that this animal possesses the three embryonic layers which correspond to those of the Vertebrates.
As his young wife's health was still too precarious to allow her to spend the winter in Russia, Metchnikoff, obliged to return to Petersburg, installed her at Montreux and asked his sister-in-law, Mlle. Fédorovitch, to stay with her. The enforced separation deeply grieved the young couple, whose only consolation was daily correspondence.
Metchnikoff resumed a life of hard work; he was now an _agrégé_ at the Petersburg University and had to leave the School of Mines; this diminished his resources, but at the same time he obtained an extra salary of 800 roubles as Extraordinary Professor. His position in the University was nevertheless very difficult, for his situation was coveted by different parties with which he had nothing to do. They wanted it for one of their adherents. His devoted friend Setchénoff, Professor of Physiology, then thought of proposing him to the Faculty of Medicine as a Lecturer in Zoology, and whilst Metchnikoff awaited the result of his efforts, he obtained leave to go to the seaside to do research work.
He joined his wife and took her to San Remo and to Villafranca. Her health had improved and she was even able to take part in his work. He was engaged in studying Medusæ and Siphonophora, animals which interested him, not only from the point of view of the origin of embryonic layers, but also from that of general morphology, for he was still pursuing the problem of genetic links between animals. He had already been able to prove the presence of embryonic layers in many inferior animals; moreover, he had found, while studying the metamorphoses of Echinodermata, the proof that the _structural plan_, hitherto considered immutable, could become transformed in course of development. Thus the bilateral plan of the larva of Echinoderma becomes a radial plan in the adult. The structural plan therefore is not an absolutely differentiating character, since specimens of the same type can show a different plan according to their stage of development. One of the genetic questions still unsolved was that of the body cavity. Always present in higher animals, it is totally absent in certain lower groups, such as Sponges, Polypi, and Medusæ. It was being questioned whether their dissimilar morphological characters did not correspond with a duality of origin separating animals which possessed a body cavity (Coelomata) from those which did not (Acoelomata).