ON THK DEVELOPMENT OF SAGITTA. 365 



body, especially when it swims. The kirvse, as a rule, lie 

 motionless near the surface, but swim in a jerky manner 

 when disturbed, just as does the adult Sagitta. In the young 

 at hatching (PI. 20, fig. 16) the tail-fin is already present to 

 some extent in all cases, but in S. en flat a there is already a 

 lateral fin beginning to be formed, continuous with the tail- 

 fin and extending forward as far as the genital cells. The 

 head is rounded and separated from the trunk by a slight 

 neck. The line separating the ectoderm from the underlying 

 mesoderm is sharply defined, and the mesoderm forms a 

 solid cord running through the length of the animal, sepa- 

 rated into two halves by the endodermic septum {end. sep.), 

 which is expanded in the head into a bulb, like a thermo- 

 meter. The ectoderm of the anterior half of the body is 

 much thickened ventral ly, forming the rudiment of the 

 abdominal ganglion {gang, v.) ; and scattered about the 

 surface, especially in the head region, are tactile organs, like 

 those of the adult, consisting of extremely fine bristles 

 arranged in a fan-like manner transversely to the length of 

 the animal {t. o.). At the base of each of these organs there 

 are groups of sensory cells, which become more obvious in 

 the later stages ; but one of them, which later is very 

 pronounced, is already noticeable just in front of the tail on 

 each side. 



Not much of the internal structure can be seen in the 

 living larvfe, but the four genital cells can generally be 

 observed embedded in the mesoderm in close contact with 

 the endodermal septum, just behind the middle of the 

 animal. They are seen better in specimens stained and 

 mounted in balsam, and these show the rudiments of longi- 

 tudinal bands of muscle just below the ectoderm. 



On the second day the larva has slightly increased in 

 length and the fin has grown larger, but has still a very 

 ragged and irregular appearance. Early on the third day 

 no pronounced change has taken place, the fin is larger and 

 more regular, long fin-rays having appeared at even intervals. 

 Tiic muscle-bands arc more developed, and show a transverse 



