323 



meantime I shall proceed to the description of the third 

 group. 



" The third group is analogous to one detected hy Dr. Farre, 

 in the ascidian zoophytes of the sea, and to which he has given 

 the name parietal. The parietal muscles take a transverse 

 course, and originate and terminate in the internal membrane 

 of the cell. In paludicella (fig. 3 and 4, k, k, k, Jc) they are 

 rather numerous, and consist of short fibres of variable length, 

 which pass transversely round the internal tunic, being 

 capiible of detection through nearly the entire length of the 

 cell, and sometimes passing one another in their course, they 

 may be seen to surround the cell with a contractile tissue. 

 Dr. Farre is of opinion, that these muscles in the zoophytes 

 which he has examined, are attached by their extremities 

 only, being free in the intermediate space. In paludicella 

 however, I saw nothing which would lead me to suspect, that 

 in this zoophyte such was their disposition. I shall not 

 here, however, speak positively, as it will require more ex- 

 tensive observations before any decisive conclusion can be 

 arrived at. 



" Such are the three great groups of muscles which I have 

 succeeded in detecting in paludicella, and so far as my obser- 

 vations have gone, analogous groups are to be found in the 

 other fresh water ciliobrachiates. It will at once be seen, by 

 any one acquainted with Dr. Farre's paper, that while the 

 muscles just described correspond in all their important 

 features with those of the ascidian zoophytes of the sea, 

 thus beautifully demonstrating the unity of type by which 

 the order is characterized, yet in the details of the several 

 groups, some remarkable modifications will be found to 

 exist. 



"The first thing which strikes us is the absence, probably 

 among all the fresh water ascidian zoophytes of that well- 

 developed fasciculus of muscular fibres, which is observed in 

 those of the sea, to arise from the bottom of the cell, and 



