'.i'2'2 .M . Ussow's Zi'(i/o(/tf<)~/'Jiiift)-i/ofo(/n'ti/ Tniu'sti(j((t!()))s. 



the Tunicata, and find in them sharply defined characters of 

 the molhiscan type. 



In my investiii;ations of the 'ruiiicuta the task T set myself 

 was, in tlio first phioe, to test by personal investigation the 

 exceedingly iniptn-tant resnlts of their develo])mcntal history, 

 and, secondly, to clear nj) as far as possible some gaps and 

 disputed questions in their anatomy. In the latter direction 

 I endeavoured chiefly to investigate : — 1, the structure of the 

 nervous system and its mode of transformation from the form 

 proper to the embryonal and larval states, which have hitherto 

 been very superfi^-ially observed and described ; 2, the intimate 

 structure of all the sensory organs of the Tunicata, which has 

 not yet been satisfactorily ascertained ; and, 3, the structure of 

 the inner and especially of the outer mantle, and of the organs 

 of circulation and nutrition, which likewise presented important 

 questions still unsolved. 



The following forms were investigated by me in different 

 directions : — 



I. Sedkxtahy Tunicata : — Asculia mammillata, intestt- 

 noUsj canina, mentula \ Cynthia microcosmus^ ^>a2>27/osa, 

 ampuloidea\ Clavellina lepadifonnis '^ BotryUus smaragdus^ 

 auratus ; Diazone violacea. 



II. Swimming Tunicata : — Apj^endieularia f areata, jin- 

 gellum, avndescens | Pyrosotna gigos ; Salpa africana — ma.v- 

 ima. deviocratica — nnicronata, runcinata — fusi'forniis, bicau- 

 daUt, jiianata] Do/iolum, Ehrenb., Nordm,, Midi. 



I now proceed to give a condensed statement of the results 

 of mv investigations. 



I. T/ie Xervous System — its anatomy, minute siriieture, 

 and mode of formation. 



All the Tunicata investigated by me (excejit the Appendi- 

 cularia) have a single un})aired ganglion (central ganglion of 

 authors, 0'1-0'15 millim. in diameter), which is both analogous 

 and homologous with the central portion of the nervous system 

 of the lower Vertebrata {Amphioxns). It is always situated 

 in the middle line on the dorsal side of the animal, close to 

 or not far from the entrance of the respiratory cavity. Both 

 the ganglion and all the jjeripheral nerves occur in the trans- 

 jjarent layer of the inner mantle, in which the muscular bundles 

 and the reticulately fibrous connective tissue (d(n-mo-muscular 

 sac) are also imbedded. The distribution of the nervous 

 dements in the ganglion is very simple and uniform. The 

 usually multi])olar nerve-cells [gymiioryta) are generally placed 

 in lav<Ms in the jieriphoral part of the ganglion, whilst its 



