INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 965 



the distribution of the nerve-fibres in the gelatinous layer, the only 

 constant character being the presence of the eight branches which 

 underlie the meridional bands. There are very various reasons for 

 regarding these elements as belonging to the nervous system : the 

 vital phenomena of these animals require it ; their histological struc- 

 ture is very similar to that of the nerves in other Invertebrata ; and 

 their mode of connection with the muscular fibres has a very striking 

 resemblance to what has been observed in the Tardigrada. On the 

 other hand, the nerves of the higher animals arise from the central 

 organs in the form of filaments of considerable thickness, and they 

 branch, as they pass peripherally, until they end in a sensory organ 

 or a muscular fibre. This is not the case with the nerve-fibres of 

 the Ctenophora ; they are branched at both ends, and the terminal 

 filaments never enter muscles, into which there only pass small 

 lateral branches. 



On the other hand, if these filaments are not nerves, can they be 

 muscles? Hardly so, for in that case Cydippe hormiphora, at any 

 rate, would have muscles of the ordinary histological character, and 

 these very special muscles also. Nor can they be a part of the 

 suj^porting system of fibres, for the fibres that have distinctly a sup- 

 I)orting function have no nuclei, and never become connected by 

 anastomosis with one another. 



We come, then, to the conclusion that the filaments which are dis- 

 tributed through the gelatinous layer of the Ctenophora have no close 

 resemblance to any elements found in higher animals, but that they 

 are best compared with the nerve-fibres of the Invertebrata. On the 

 physiological side their nervous function is very distinctly spoken to. 



Helaiions of the Ctenophora to the other Cuelenterata. — The author 

 regards these forms as being very distinct from tbe rest of the group 

 in which they are placed. To decide the questions which have been 

 raised with regard to this subject, we have, first of all, to inquire 

 whether in the developmental history of the higher animals there is 

 any stage which is comparable to that which is permanent in the 

 Ctenophora. The nervous system of most of these, at any rate, is 

 situated in the mesoderm ; the same is true of these Coelenterates. 

 But now it has to be seen whether this system, derived in both cases 

 from the ectoderm, is in the first stage scattered through the mesoderm 

 and only secondarily concentrated. This, of course, is not the case. 

 Where the nervous system remains ectodermal in position, as it does 

 in Some of the higher Metazoa, it is nevertheless even there concen- 

 trated. Eimer's hypothesis is hereby negatived. 



Secondly, there arises the questicjn, what relation have the neuro- 

 muscular cells of Beroii to those of Uijdra > The only point of resem- 

 blance is that mode o"f continuous connection between muscle and 

 nerve wliich always occurs in all animals provided with these structures; 

 otherwise there is nothing in common. The ueuro-muscidar cell of 

 Jli/dra is an cctodeniial, the neuro-muscular fibre of Bcror an eudo- 

 deruuil structure. The sensory cells only become connected with the 

 latter in a secondary fa-hiun. 



The author then shows how the ucuro-muscular systems in the 



