ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 57 



cells to definite maturation-centres ; but the resemblance is not complete, 

 inasmuch as the cells in the polyp are already differentiated into 

 generative cells when they begin to wander, while in Echinoderms the 

 differentiation is effected after the migration. 



True Nature of the Madreporic System of Echinodermata.* — 

 Prof. M. M. Hartog comes to the conclusion that the rnadreporic system of 

 Echinoderms is morphologically and ontogenetically a (left) nephridium. 

 He has found by experiments that its ciliary current is directed outwards 

 through the madreporite, and that in Comatula an outward current takes 

 place through the pores of the disc. As against the theory that the 

 system serves for taking in water, the author urges that there is no need 

 for this since osmosis is amply sufficient for the turgescence of dilatable 

 organs. The rapid contraction or erection of the tube-foot is due to the 

 transference of liquid from one part to another. The change of position 

 of the rnadreporite in most Holothurians is, it is suggested, probably 

 due to the usurpation of nephridial functions by the respiratory tubes 

 which are connected with the cloaca. 



The author takes the opportunity of remarking that it is very 

 probable that, when an Actinian is at rest, the oral slit is completely 

 closed ; turgescence of the body is effected by osmosis, and the apical 

 pores of the tentacles would appear to have the double function of the 

 periodical or perhaps constant discharge in small quantities of the excess 

 of liquid, and of its rapid discharge when, in defence, the animal wishes 

 rapidly to reduce its bulk. 



Nervous System and Vascular Apparatus of Ophiurids.f — M. S. 

 Cuenot has examined the nerve-trunks of Opkiurids after treatment with 

 osmic acid and distilled water, and finds that they are formed of an 

 epithelium of elongated cells, among the bases of which very fine nerve- 

 fibrils run. The epithelial nuclei are all placed above the fibrils, and 

 it is they which were taken by MM. Teuscher and Koehler for nerve- 

 cells. The histological characters of the nerve-trunks of Opkiurids 

 are, then, exactly the same as those of Asterids. The nervous ring, in 

 addition to the ambulacral nerves, gives off two branches in each inter- 

 radius ; the more external of these goes directly to the large external 

 interradial muscle, and the other, which is larger, gives branches to the 

 dental papillae. In the Ophiurids which were examined the oesophagus 

 was found to be directly continuous with the nerve-ring by a delicate 

 membrane in which nuclei are scattered ; in Asterids the two are in 

 more obvious connection. In the Euryalidse the oesophagus receives 

 numerous nerves, united into a plexus, which becomes united with the 

 nerve-ring. 



Branches from the radial nerves penetrate the ossicles of the arm 

 and terminate in the intervertebral muscles, which are the active agents 

 in locomotion. The branches distributed to each sjnne have each a small 

 swelling formed by nerve-cells or fibres ; they extend some way along 

 the axis of the spine, and then become lost in its substance. 



The circular and radial vessels which MM. Ludwig and Koehler 

 have called the vascular system are only connective-cells and fibres, and 

 have no morphological value. There is a supraneural sinus (the peri- 

 hsemal of Ludwig and Koehler), within this a nerve-trunk, then a vascular 



* Ann. and Mag. Nat. Hist., xx. (1887) pp. 321-G. 

 t Coinptes Rendus, cv. (1887) pp. 818-20. 



