THE ENTEROPNEUSTA. 6 



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 larvae are to be found in Stage Gr 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 of a leech. The appear- 

 ance of this organ bears some resemblance to the terminal 

 sucker described by Graaf as occurring in certain Rhabdo- 

 coeles. 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 

 same age without it ; hence it may be inferred that it undergoes 

 a rapid atrophy at this point. 



