LANCELET. d 



For the general internal structure of the Lancelet the visitor is 

 referred to the special case affixed to the side of one of the arches 

 on the West side of the Entrance Hall of the Museum. 



The two specimens A and B, 10-18, mounted on the framed pane 

 of glass in Table-case 21 are wax models, enlarged 100 diameters, 

 showing the remarkable lack of symmetry that exists during the 

 early or larval stages of the Lancelet. The mouth developes on the 

 left side, and only subsequently passes downward to occupy a median 

 position. Primary gill-slits, to the number of fourteen, appear 

 in the ventral median line and move upwards on the right side. 

 Eight secondary gill-slits then appear above them on the right 

 side, and, as they enlarge, the primary gill-slits descend and pass 

 across to occupy their permanent position on the left side. After 

 this, tertiary gill-slits develope on both sides behind the existing 

 ones and continue to increase in number throughout life. The 



Fig. 1. — Lancelet : a, mouth ; b. atriopore ; c, vent or anus ; 

 d, anterior end of notochord. 



three models 1019 C, D, and E on the other framed glass show 

 the manner in which the atrial wall, which in the adult covers 

 over the gills, closes by the downward and horizontal extension of 

 the side folds of the body (metapleural folds), and the union of the 

 edges of these flaps along the middle. The models represent 

 three different stages of development, and show the ventral or 

 under surface of the larva. In C there is no atrial floor, the right 

 and left metapleural folds being distinct ; in D the horizontal 

 ledges growing in from the inner faces of the metapleural folds 

 have united in the hinder part ; in E the whole of the atrial floor 

 is complete except at the extreme anterior end. 



On the other side of the Table- case are shown enlarged models 

 of Tunicates for comparison with the Lancelet, one (1050) of the 

 larva of a simple Ascidian such as the common Sea-squirt, and 

 three models (1051, 1052, 1053) showing different views of an 



b2 



