OBSERVATIONS ON POLYZOA. 105 



ever, observed and figured it when expanded and having 

 the peculiar spindle-shape represented below in Figs. 10 

 and 11. The contraction of the nerve-mass in Freder- 

 icella invariably takes place on the ventral side and to- 

 wards the posterior end, and when in this state it is gen- 

 erally removed to some distance from the oesophagus. 

 These movements of contraction and expansion occur very 

 irregularly, and often when the former happens, the entire 

 mass is drawn up into the cavity of the epistome. This 

 generally precedes the invagination of the polypide in Plu- 

 uiatella, and the epistome, in all other genera, doubtless 

 serves, Avhile within the coenoecial cell, to protect the ner- 

 vous centres from compression by the surrounding mem- 

 branes. During the confinement of the polypide in the 

 cell, the epistome lies in the hollow formed by the overarch- 

 ing bases of the tentacles, and is admirably fitted to receive 

 and guard them from injmy . No muscles were detected in 

 connection with the ]]erve-niass, which could have been 

 instrumental in occasioning either the contractions or the 

 subsequent elevation of this part into the cavity of the 

 epistome. 



Notwithstanding its mutability there is every reason 

 for regarding this body as a true nerve-mass. The nu- 

 merous branches from the sides, the angular junctions of 

 these with the main body, and the evident solidity of the 

 whole, leaves no doubt of its nervous nature. The prin- 

 cipal nerves spring from the sides (Fig. 9), and are divis- 

 ible into anterior and posterior sets according to their places 

 of origin. The posterior set comprises the nerves distrib- 

 uting themselves to the epistome ,lophophore, tentacles and 

 evaginable endocyst, and the anterior set those which go to 

 the various parts ot the alimentary canal. For conven- 

 ience sake I have designated the larger branches and their 

 smaller ramifications, respectively, by the names of nerve- 

 trunks, and nerve-branches. The anterior set all arise from 

 four large nerve-trunks near the posterior end of the nerve- 

 mass, two on either side (Figs. 10, 11) . Two of these, the 

 branches of which are unknown, supply the epistome, and, 

 perhaps, the oral portion of the oesophagus. They are trace- 



COMMUNICATIONS ESSEX INSTrrUTE, VOL. V. 15. XOV. 22, 1866. 



