DEVELOPMENT OF BALANOGLOSSUS KOWALEVSKII. 83 
The external relations of the parts then become changed 
until finally the larva has the shape shown in No. 3, figs. 16 
and 17. In this stage the proboscis is conical and the middle 
segment much shortened. In the anterior dorso-lateral region 
of the third segment are the openings of the first pair of gill- 
slits, which are simple circular pores leading into the 
archenteron. 
The larva is still opaque, and pale yellowish-brown in colour. 
In this condition it remains for about ten days, at the end of 
which time the second pair of gill-slits is formed. The body 
has become partly transparent, especially in the region of the 
proboscis, through the walls of which muscle-fibres are visible. 
Other external changes which occur at this period are the loss 
of the anterior tuft of cilia and the gradual disappearance of 
the posterior ciliated ring. At the commencement of this 
period the larve are to be found in Stage G at a depth of 
about six to eight inches in the sand, but towards its close they 
work their way into the higher strata of mud, and do not again 
go down again until the adult condition is reached. 
As the cilia disappear a peculiar organ is formed as a small 
papilla, bearing long cilia and mucous glands, situated at the 
central part of the posterior surface (fig. 1, sk.). This organ 
serves as a sucker, by which the animal can attach itself to 
foreign bodies sufficiently firmly to prevent itself being washed 
off by a stream of water from a pipette. The anterior surface 
of the proboscis is also slightly suctorial, and by thus fixing 
itself posteriorly and extending the proboscis it is able to creep 
slowly about, somewhat in the manner ofaleech. The appear- 
ance of this organ bears some resemblance to the terminal 
sucker described by Graaf as occurring in certain Rhabdo- 
ceeles. It subsequently attains a considerable size, and is 
traversed by several wrinkles (figs. 3 and 4, sk.). This organ 
afterwards entirely disappears, but as to its mode of disappear- 
ance I have no certain observations. It would appear to occur 
very suddenly at the stage when the animal possesses seven to 
eight gill-slits. I have found animals with eight gill-slits 
which possess this sucker, and also animals of apparently the 
