514 SUMMARY OF CURRENT RESEARCHES RELATING TO 



which is always formed by the same cuticle as that of the wall of the 

 cupola. These tubes traverse the chitinous layer, becoming more 

 delicate as they approach its internal surface, but their lumen always 

 remains perfectly distinct ; they take a slightly sinuous course. It 

 is by these tubes that the cupolse receive the nutrient .materials 

 which they require. 



The chitinous layer of the peduncle of Anatifer has no special 

 covering, but the cuticle-membrane has certain thickenings which, to 

 some extent, recall the arrangements which are observed in PolUcipes. 

 These are hemispherical swellings with globules differentiated in their 

 interior ; these latter are formed by the cuticle, and are received into 

 pits of the chitinous layer. The formations are more complicated in 

 PoUicipes, but in both genera they are continuous with similar fibres 

 which traverse the subjacent chitinous layer in radiate fashion. The 

 structure of the calcareous valves of the capitulum also presents some 

 peculiarities in PoUicipes, for the valves are of considerable thickness, 

 and the calcareous plates are divided into three or four strata by 

 secondary layers of the cuticle. The general tissue of the plates is not 

 compact as in other genera, but contains numerous lacunse, which are 

 absolutely empty. 



Vermes. 

 a. Annelida. 



Influence of Nervous System of Annelids on Symmetry of the 

 Body.* — M. L. Eoule, who has studied the development of various 

 Annelids, and especially of the Enchytrseidae, attempts a sketch of the 

 development of the nervous system. The nervous centres are of 

 epiblastic origin, and the first is the cephalic or frontal plate ; it alone 

 exists in those embryos in which the development is condensed, but it is 

 not so with larvse. These latter also have a subepiblastic nervous 

 plexus which is chiefly placed under the vibratile oral corona ; it some- 

 times, as in LopadorTiynchus, becomes a compact ring. We have here a 

 radially symmetrically nervous system, and the embryos are oval or 

 spherical. The annular plexus is peculiar to the larva, and disappears 

 after the larval stage. The body next elongates and a third nervous 

 rudiment arises in the metasoma ; this is the future ventral nerve-cord. 



At their first appearance the plates are merely local proliferations of 

 the ectoblast, which are thicker in the centre than at the sides. Changes 

 occur when the mesoblast begins to be developed, for this elongates with 

 the growth of the body, and the primitive radial symmetry is converted 

 into the bilateral, which is preserved in the adult ; the rudiments of the 

 nervous centres are modified to follow this change of symmetry. Two 

 chief centres of proliferation appear in each of the cephalic and medullary 

 plates, and are arranged symmetrically around the new longitudinal 

 axis which divides the body into two halves. 



In the primitive types the medullary cords inclose, for the whole of 

 their extent, an equal number of nerve-cells and fibrils, while in the 

 higher types there is a differentiation into ganglia formed of cells only 

 and of connective fibrils. The author cannot regard the phenomenon of 

 the production of the metasoma by the prosoma as similar to an alterna- 

 tion of generation, as does Kleinenberg. Nor, as he will show in a 



* Comptes Kendiis, cviii. (1889) pp. 359-61. 



