THE LATERAL LINE ORGANS. 29 



dark, like the former fibres, and yet of small size. They 

 are not, however, so small as the ordinary communis 

 fibres. 



J. — T/ie Extra-scapular Canal. 



i. — TJie Post-Occipital Main Canal. — Just cephalad of 

 the pectoral fin the canal system of the head appears, as 

 the direct continuation of the lateral line of the body. As 

 we pass cephalad this canal is first a dermal groove which 

 very soon closes to form a narrow canal in the dermis and 

 the latter at once sinks down into a gi:oove in the extra- 

 scapular bone. At the level of the last sense organ of the 

 head (715 on the plots) the bony groove has become a 

 canal. In this osseous canal the membranous canal ex- 

 pands to the normal width, which is maintained with 

 tolerable uniformity throughout the entire canal system 

 of the head. 



The sense organ above referred to (w. j of Fig. 5) is 

 the only one of the main line behind the occipital com- 

 missure which is enclosed in a canal. It is supplied by 

 the first ramus of the r. lateralis vagi (r. /. /.), excluding 

 the r. supra-temporalis. This organ might be regarded 

 either as the first one of the trunk series or the last one of 

 the head series. 



a. — TJie Occipital Commissure. — The incomplete occipi- 

 tal commissure branches off from the main canal at an 

 acute angle and extends dorsad and cephalad only about 

 one mm. before it opens out and disappears. A single 

 elongated canal organ is found in this canal (Fig. i, ni. 4). 

 It lies very near the origin of the canal and is supplied 

 by a branch of the r. supra-temporalis vagi. No naked 

 buds could be found in the skin along the line continuing 

 the course of the canal, as is the case behind the lateral 

 line canal. 



After its separation from the main canal the occipital 

 commissure remains in the extra-scapular bone. The 

 main canal, however, soon leaves this bone, lies close 

 under the skin and immediately opens out for a short dis- 



