(317) C. Claus. "Studien üb. Polypen u. Quallen d. Adria. 1. Acalephen, Discomedusen," _Denk. d. math.-naturwiss. Classe d. k. Akad. Wiss. Wien_, Vol. XXXVIII. 1877. (318) Th. Eimer. _Zoologische Studien a. Capri. 1. Ueber Beroë ovatus. Ein Beitrag z. Anat. d. Rippenquallen._ Leipzig, 1873. (319) V. Hensen. "Zur Entwicklung d. Nervensystems." _Virchow's Archiv_, Vol. XXX. 1864. (320) O. and R. Hertwig. _Das Nervensystem u. d. Sinnesorgane d. Medusen._ Leipzig, 1878. (321) O. and R. Hertwig. "Die Actinien anat. u. histol. mit besond. Berücksichtigung d. Nervenmuskelsystem untersucht." _Jenaische Zeit._, Vol. XIII. 1879. (322) R. Hertwig. "Ueb. d. Bau d. Ctenophoren." _Jenaische Zeitschrift_, Vol. XIV. 1880. (323) A. W. Hubrecht. "The Peripheral Nervous System in Palæo- and Schizonemertini, one of the layers of the body-wall." _Quart. J. of Micr. Science_, Vol. XX. 1880. (324) N. Kleinenberg. _Hydra, eine anatomisch-entwicklungsgeschichtliche Untersuchung._ Leipzig, 1872. (325) A. Kowalevsky. "Embryologische Studien an Würmern u. Arthropoden." _Mém. Acad. Pétersbourg_, Series VII., Vol. XVI. 1871. (326) E. A. Schäfer. "Observations on the nervous system of Aurelia aurita." _Phil. Trans._ 1878. Nervous system of the Invertebrata. Our knowledge of the development of the central nervous system is still very imperfect in the case of many Invertebrate groups. In the Echinodermata and some of the Chætopoda it is never detached from the epidermis, and in such cases its origin is clear without embryological evidence. In the majority of groups the central nervous system may be reduced to the type of a pair of cephalic ganglia, continued posteriorly into two cords provided with nerve-cells, which may coalesce ventrally or be more or less widely separated, and be unsegmented or segmented. Various additional visceral ganglia may be added, and in different instances parts of the system may be much reduced, or peculiarly modified. The nervous system of the Platyelminthes (when present), of the Rotifera, Brachiopoda, Polyzoa (?), the Mollusca, the Chætopoda, the Discophora, the Gephyrea, the Tracheata, and the Crustacea, the various small Arthropodan phyla (Poecilopoda, Pycnognida, Tardigrada, &c.), the Chætognatha (?), and the Myzostomea, probably belongs to this type. The nervous system of the Echinodermata cannot be reduced to this form; nor in the present state of our knowledge can that of the Nematelminthes or Enteropneusta. It is only in the case of members of the former set of groups that any adequate observations have yet been made on the development of the nervous system, and even in the case of these groups observations which have any claim to completeness are confined to certain members of the Chætopoda, the Arthropoda and the Mollusca. An account of imperfect observations on other forms, where such have been made, will be found in the systematic part of this work.