212 WILLIAM CATESON. 



has appeared (fig. 12, g.) dividing the body into two nearly- 

 equal parts. Subsequent changes show that this groove marks 

 off the region which is to form the proboscis from the rest of 

 the body. 



As this groove increases in depth a second groove is formed 

 behind it (fig. 13^ c. g.). The area between these two grooves 

 forms the collar. The proportions of the body have also 

 altered considerably; the length of the proboscis being now 

 only about a third of the total length of the animal. At this 

 stage arises also a tuft of long flagelliform cilia at the apex of 

 the proboscis. No eye-spots are present, however, at this or at 

 any subsequent stage. 



The animal remains in this condition for some hours and is 

 generally hatched without the occurrence of any further 

 alteration. The time of hatching is, however, quite irregular. 

 Larvse may frequently be found swimming freely whose organi- 

 sation is not much in advance of Stage C, and on the other 

 hand, I have seen them in the condition of Stage G still 

 enclosed in the eggshell. 



While in the eggshell from Stage C onwards the larva swims 

 about very rapidly, rubbing the membranous shell with its 

 anterior end until it gives way, and the animal escapes. On 

 leaving the egg it does not swim at the surface as pelagic 

 larvse do, but creeps about in the mud, burrowing with its 

 proboscis, in the walls of which muscle fibres soon appear 

 (v. infra), and also propelling itself by means of its ciliated 

 band. If placed in a beaker of water it sinks to the bottom at 

 once. Two specimens of Stage H were, however, taken on 

 one occasion in the surface-net, but from the molluscs and 

 other creatures present with them, it was clear that the net 

 had been allowed to drag along the bottom. As the water was 

 never more than four feet deep at high water this was easily 

 possible, and occurred continually at Hampton. 



Stage F. — The next feature of importance which generally 

 occurred shortly after hatching, was the appearance of a longi- 

 tudinal groove in the middle dorsal line of the collar (fig. 

 14, n g). This groove is a temporary structure, only lasting 



