NERVOUS SYSTEM. 335 



thesis fits in moreover very well with the condition of the neuro-muscular 

 system as we find it in the Ccelenterata, 



Bibliography. 



Origin of the Nervous System. 



(316) F. M. Balfour. "Address to the Department of Anat. and Physiol, of the 

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(317) C. Clans. "Studien iib. Polypen u. Quallen-d. Adria. I. Acalephen, Disco- 

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 1877. 



(318) Th. Eimer. Zoologische Shidien a. Capri. I. Ueber BeroS ovatus. Eiii 

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{319) V. Hensen. "Zur Entwicklung d. Nervensystems." Virchow^s ArcMt\Yo\. 

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(320) 0. and R. Hertwig. Das Nervensystem u. d. Sinnesorgane d. Medmen. 

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(3'2i) 0. and R. Hertwig. "Die Actinien anat. u. histol. mit besond. Beriick- 

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(322) R. Hertwig. "Ueb. d. Bau d. Ctenophoren." Jenaisclie Zeitschrift, Vol. 

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(323) A. W. Hnbrecht. "The Peripheral Nervous System in Palaso- and Schizo- 

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(324) N. Kleinenberg. Hydra, eine anatomischrenttcicklungsgeschichtliche Un- 

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(325) A. Kowalevsky. "Embryologische Studien an Wiirmem u. Arthropoden." 

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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 Chsetopoda 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 pos- 

 teriorly 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 Mol- 

 lusca, the Chaatopoda, the Discophora, the Gephyrea, the Tracheata, 

 and the Crustacea, the various small Arthropodan phyla (Poecilopoda, 

 Pycnognida, Tardigrada, &c.), the Chgetognatha (?), and the Myzo- 

 stomea, 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 observa- 

 tions which have any claim to completeness are confined to certain 



