6 MORPHOLOGY OF 



contracted condition of the body is a very marked structure 

 (fig. 3, op.). As development proceeds, this thickening in- 

 creaseSj and at the same time grows backwards in the lateral 

 regions until (five gill-slits) it has covered half the first gill- 

 slit. The degree of contraction of the body of course alters its 

 relations, but in the extended condition in adult specimens its 

 posterior margin is about on a level with the fourth gill-slit. 

 As then this structure is a process of the body wall which 

 forms an opercular fold over the foremost gills, it appears 

 reasonable to institute a comparison between it and the atrial 

 walls of Amphioxus, &c. It will therefore be alluded to as the 

 operculum, and the cavity between it and the body wall as 

 the atrial cavity. 



As previously described, the first gill-slit on its appearance 

 is a simple circular pore. In this condition it remains for 

 some days, but gradually, on about the tenth day, its form 

 changes owing to the growth of a process from the dorsal 

 margin of the pore, which renders the aperture somewhat 

 kidney-shaped (fig. 1). Whilst this process is continuing the 

 second gill-slit appears as another circular pore, similar to the 

 original aperture of the first gill-slit. I was unable to discover 

 any priority in the appearance of the gill-slit of either side in 

 particular, but incline to believing that they appear syn- 

 chronously on the two sides of the body, as previously 

 mentioned. 



After the appearance of the first gill-slit a definite series of 

 changes is undergone, and these form a definite period in the 

 history of the development of the animal, which may be con- 

 sidered as closing with the formation of the second pair of gill- 

 slits. 



At this point it may be well to recapitulate these changes. 

 The embryonic life terminates with hatching, and the free 

 larva with one pair of gill-slits escapes, moving about in the 

 mud by means of cilia. 



Before the second pair of gill-slits appear, it has undergone 

 the following changes : 



