A new Species of Nectoueraertes. 341 



A clearly defined layer of diag'onal muscles has no existence in 

 this species. At the extreme anterior end of the animal there are 

 indications that such fibres are present but in almost every case 

 they may be seen to be in reality longitudinal bands, which here 

 are considerably curved, or they are continuous with fibres attaching 

 to the proboscis. 



The longitudinal layer in the head is relativly thin (0,02 mm) 

 but a short distance anterior to the cirri it becomes well developed 

 dorsally and ventrally save in the mid line (Fig. 13, 16) where 

 throughout the entire body anterior to the base of the tail it remains 

 comparativly undeveloped. In the region of the cirri the longitudinal 

 muscles along the sides of the body increase in thickness and become 

 continuous with the circular muscles of the cirri (Fig. 18). Posterior 

 to these last named organs the longitudinal muscle sheet assumes 

 a condition of affairs represented in Fig. 13 which continues to the 

 tail region with a few comparativly slight modifications. As the 

 posterior end of the body is approached the fibres of the mid line 

 become as abundant as in the adjoining regions; but on the other 

 hand in the lateral regions, which are abruptly thinner especially 

 posteriorly, the longitudinal fibres become reduced to a thin sheet. 

 This is especially well shown in Fig. 16 which represents a section 

 taken a short distance in front of the caudal peduncle. In the 

 narrow region immediately in front of the tail fin the keel-like 

 projection is lacking and the longitudinal layer is of uniform thickness 

 throughout (Fig. 14), This last statement applies with equal force 

 to the caudal fin for although the longitudinal muscles form a thinner 

 sheet than more anteriorly they nevertheless constitute a layer of 

 uniform thickness (Fig. 20). 



Dorso-ventral fibres occur throughout the entire body. In the 

 head they are comparativly few and slender but in the intestinal 

 region they become somewhat more numerous and of larger size and 

 as usual alternate with the intestinal lobes. At certain points some 

 of their fibres attach to the oesophagus and stomach and others aid 

 in the movements of the proboscis though this in addition to its 

 intrinsic musculature is operated by numerous radial fibres. In the 

 lateral margins of the body which become thin, and in the tail fin 

 and especially in the cirri dorsoventral fibres become numerous and 

 are frequently of considerable size. 



Regarding the histology of this system little need be said. 

 Each fibre is a single cell and contains a well developed elliptical 



22* 



