ON A TOENAEIA FOUND IN BRITISH SEAS. 65 



length ; but in this respect there is great variety among individuals. 

 The pre-oral and longitudinal bands have undergone considerable 

 changes both in their arrangement and in histological characters. 

 In the earlier larva the pre-oral band was triangular, the base over- 

 hanging the mouth, and the apex touching the apical sense organ. 

 In this stage the external angles of the triangle are drawn out on 

 either side into long loops which are turned upwards towards the 

 apex, the remainder of the figure appearing as a third median loop 

 which touches the sense organ. The lower edge overhanging the 

 mouth has not undergone any change. The longitudinal band 

 forms a sort of cross the head of which touches the sense organ, 

 the arms are turned upwards like those of pre-oral band. Ventrally 

 the angle beneath the mouth is larger and more sharply defined, 

 but otherwise no important change has taken place. In section, 

 both the pre-oral and longitudinal bands are seen to be composed of 

 numerous densely-crowded, deeply-stained nuclei, the cell outlines of 

 which are not distinguishable. In the earlier stage the cells of the 

 bands were columnar and fewer in number. The cilia have nearly 

 disappeared in the later stage, but a few fine patches may be dis- 

 tinguished. The circumanal band on the other hand has increased 

 in size and importance, and is now the chief if not the only organ 

 of locomotion. Its long and powerful cilia are borne on long 

 columnar cells arranged in oblique rows of three, as is shown in 

 fig. 11. Above and below the ciliated cells are supported by 

 packing cells, and numerous spots of bright brown pigment mark 

 the course of this as well as of the pre-oral and longitudinal bands. 

 Each ciliated cell is long and columnar, slightly contracted in the 

 middle of its length, with a large nucleus. The cilia can be traced 

 as fine fibrillae inwards as far as the nucleus, but I could not deter- 

 mine whether they entered the nucleus or not. 



The alimentary tract does not require a detailed description. 

 The chief distinction between this stage and the last consists in the 

 large circular aperture between the mid-gut and hind-gut. The anus 

 remains relatively small. 



The anterior body-cavity, which from its fate may conveniently be 

 spoken of as the proboscis cavity, shows the same relations as in the 

 previous stage. The whole cavity is larger, and its walls are 

 further differentiated, the anteinor wall being considerably thickened. 

 Between the thickened portion and the point of insertion of the 

 oesophagus on the mid-gut a few scattered cells may generally be 

 distinguished. 



The " heart '' of Agassiz, Metschnikoff, and Spengel makes its 

 appearance at this stage as a vesicle lying just above and to one 

 side of the proboscis pore. Figs. 6 — 10 are a series of sections 



VOL. II, NO. I. 5 



