AN UNDESCRIBED A CRANIATE. 223 



starting from the pre-oral hood they run back parallel with one 

 another as far as the atriopore, thus forming the lateral limits of 

 the floor of the atrium. They do not stop at the atriopore, but as 

 seen in Figs. 1, 2, 3, 4, 5, continue posteriority, the left one but a 

 short distance, the right one, however, becoming continuous with 

 the median ventral fin. This continuity of the median ventral fin 

 and the right metapleuron can be seen in surface views (Figs. 15), 

 and also demonstrated in serial sections. 



Such sections show that the fin retains its solid structure up to 

 point anterior to the atriopore where it, or the metapleuron as 

 we may now call it, acquired the usual hollow condition known 

 in other lancelets. The left metapleuron likewise is solid and 

 identical with a fin in structure at its posterior end. In a section 

 across the middle of the body (Fig. 14) we see two hollow metapleura, 

 but at the level of the atriopore (Fig. 1 5) there are two fins, the left 

 the smaller. Following a few sections forward from the atriopore 

 the thick connective mass of the fins is found to continue as the 

 thick sub-epidermal layer on the outer side of the metapleura, 

 while a new cavity, the large lymph space of the metapleura, 

 gradually extends downward from the basal part of each fin, or 

 metapleuron. 



This continuity of the right metapleuron with the median fin is 

 unlike the condition found in the European Amphioxus and leads to 

 an interesting departure of the fin from its true median position. In 

 passing forward from the tail process the median fin departs de- 

 cidedly to the right, even before the anus is reached, and becomes 

 attached along the right edge of the ventral aspect. 



Where the anus opens out on an anal papilla (Fig. 22) the fin 

 is on the right side. We see here that though the digestive tract 

 discharges to the left of the median fin it is not at all asymmetrical 

 with reference to the median plane of the animal : it is the fin 

 which is placed on the right, while the digestive tract remains a 

 median structure. Yet the actual orifice of the rectum may, as in 

 the figure, be turned to one side and lie to the left of the median 

 plane; the papilla with its sphincter muscles is not accurately 

 bisected by the median plane, but this does not justify us in regard- 

 ing the anus as morphologically out of the median plane. 



