SOME POINTS IN THE ANATOMY OF POLYCHvETA. 273 



ration. It may be pointed out here that the nerve-cords in 

 Sabella are not separated from the epidermis, while in Myxi- 

 cola they are completely so. 



We pass now to consider another family in which colossal 

 tubes or neural canals are greatly developed, the Spionidse. 

 In Nerine coniocephala, Johnston (fig. 23), the nerve- 

 cords are differentiations in a thickened epidermis, less dis- 

 tinctly defined from the surrounding cells than is the case in 

 Sabella. In the median line between the two cords is an 

 enormous neural canal, larger in sectional area than the two 

 nerve-cords together, but having a structure similar to the 

 tubes in Sabella. The appearance of this canal in section is 

 seen in fig. 23. The canal contains a shrunken, gelatinous- 

 looking mass as, in other cases. On tracing this canal for- 

 wards it is found to become smaller near the anterior end, and 

 to cease altogether much sooner than is usually the case. I 

 have found no indication of its anterior division into two 

 canals. The last trace of it seen in approaching the head is 

 shown in fig. 24. 



In Scolecolepis vulgaris, Malmgren, there are two 

 neural canals, one on the inner side of each nerve-cord. 



In Magelona, which forms a family by itself, there is a very 

 large median neural canal resembling that of Nerine, but lying 

 below the nerve-cords instead of above them (fig. 25). 



In the Ariciidae I need only confirm the account given by 

 Mcintosh, that in the middle of the body the nerve-cords are 

 thrust inwards by the great ventral longitudinal muscles, 

 which contain between them a narrow lamina preserving the 

 connection between the nerve-cords and the epidermis. A 

 single median neural canal runs above the nerve-cords as in 

 Nerine. 



In Arenicola ( Telethusidae) the nerve-cords are separated 

 from the epidermis by the layer of circular muscles ; there is a 

 small neural canal, entirely filled with a homogeneous mass, at 

 the dorsal and inner side of each cord. 



In Trophonia (Chlorhsemidse) Mcintosh does not mention 

 the existence of neural canals, but one of these exists in each 



