'i)'24 .M. U:*sow's Zoologico-Einhtyologicat I/irt'sfu/afiuiix. 



The complete absence of the oesophageal nervous ring whicli 

 exists in the Molhisca, and indeed is characteristic of them, 

 the unity in tlie structure of their central ganglion, and the 

 development of all parts of tlielr central nervous system from 

 the upper germ-lamella in the form of a nervous rin^ becoming 

 segmented into three parts are facts which decidedly negative 

 the homology en-oneously ascribed to them (the so-called sipho- 

 nal ganglion in Teredo navah's*). 



From a comparative revision of the nervous system in the 

 different species of Tunicata, the following conclusions may 

 be drawn : — in the AppendiciiJaria the plan of stnicture of 

 the nervous system is in some degree like that of the Ascidia ; 

 the neiTOUs system of the Pyrosomata may be regarded as a 

 transition form between the transformed nervous system of 

 the adult Ascidia, and the type of stnicture of the nervous 

 system of the Saljxv and Cyclomyancp. 



The process of transformation of the nervous system of the 

 Ascidian larvae commences immediately after their attachment 

 (so-called sessile form). The ganglion is formed by multi- 

 plication of the embryonal cells, which chiefly occupy the 

 lower part of the upper sensory vesicle and the upper part of 

 the tiamk-vesicle. The caudal part ("dorsal cord") of the 

 embryonal nerve-tube is atrophied without leaving any traces. 

 The pigment of the visual and auditory organs, and all other 

 parts of the dissolving nervous system of the Ascidian larva, 

 become converted into fat-drops, which are gradually absorbed 

 by the young nerve-cells, with which the narrowing cavity 

 of the nerve-vesicle is more and more filled. The formation 

 of the blood- corpuscles is not dependent upon the above- 

 mentioned metamorphosis of the vanishing embryonal system f. 

 In this way the transformation of the embryonal nervous system 

 into a central ganglion is effected. The ganglionic membrane 

 is developed from the outer cell-layer of the young ganglion. 

 At the time of the formation of the branchial fissures the 

 ganglion is already almost completely developed. Numerous 

 processes of cells of the nervous vesicle, which are at first 

 spherical but gradually elongate and divide, gradually fill up 

 its original cavity. The development of the peripheral nerves 

 is effected by means of a chain-like coalescence of individual 

 nei-ve-cells whicli occupy the inner mantle. The finely gran- 

 ular protoplasm of these cells may be regarded as the original 

 substance from which the fibrillar axial cylinders of the nerve- 

 threads are formed. The stellate cells of the connective tissue 

 form by coalescence the neurilemma of the above-mentioned 

 nervous bundles. 



♦ \o\\ Baer. loc. cit. p. 21. t Mull. Aich. Uo-J, p. 317. 



