No. 2.] LINE SYSTEM OF BATRACHUS TAU. 245 



The canals are formed in sections, as described by Allis (2) : 

 " After a developing canal organ has reached the surface, it 

 begins to sink, carrying with it the surrounding tissues, thus 

 forming a small pit at the bottom of which the organ lies. 

 Lips grow upward and inward from the edges of the pit, and 

 meeting above the organ, form a short canal, the openings of 

 which are inclined to the general surface and give to the canal 

 a tunnel-like appearance." In Figs. 7 and 8 the organs have 

 begun to sink below the level of the surface and form linear 

 areas of depression. 



In Figs. 9, 10, and 11 the process has been continued and 

 the organs are partially enclosed by the approaching lips of the 

 canal, but complete fusion has not taken place. This condition 

 is permanent in some forms, as Chimaera and Polyodon, 

 open grooves taking the place of canals in the adult. The 

 process of enclosure goes on unequally; the most anterior organs 

 are the first to become enclosed. In Fig. 10 the line of fusion 

 of the nasal tube is distinctly seen, and the two half pores 

 which are formed constitute the anterior and posterior nares. 

 In the supraorbital line the process of fusion is carried out 

 most completely, the short canals coalescing and therefore no 

 primary pores formed, the terminal or half pores only being 

 present (Figs. 2 and 23). A comparison of the commissural 

 canal between the eyes, so prominent at this stage (Fig. 10), 

 with the bony channels on the frontal bones (Fig. 4) is instruc- 

 tive, as showing the effect of the flattening of the head and 

 the closer approximation relatively of the eyes in the adult. 



In the case of the operculo-mandibular line (Fig. 9) the oper- 

 cular portion is seen to form independently of the mandibular 

 division, and the double or primary pore which marks their 

 union remains larger than the others of the line (Fig. 22). 

 In the mandibular portion of the line the four anterior organs 

 are never enclosed in a canal, but retain the open groove 

 condition in the adult (Fig. 3). 



4. Connecting strand. — While examining adult specimens of 

 Batrachus which were partially macerated in nitric acid my 

 attention was attracted by a very well-defined strand connecting 

 the organs on the side of the body. This structure had the 



