372 L. DONCAS'TER. 



posterior end of the g-ut to the transverse septum. The 

 space is in the ventral half of the body ; dorsally the longi- 

 tudinal mesentery extends continuouslj'' from the trunk into 

 the tail. This arrangement is also found in the adult 

 Sagitta, but is most pronounced in S. lyra, in which the 

 distance between the anus and the transverse septum is 

 considerable. When the anus is formed a swelling can 

 generally be seen in the rectal portion of the gut, just in 

 front of it, and the endodorin cells develop cilia, which are 

 especially active in this region. Small particles can be seen 

 circulating in the distended rectum, so that the young 

 animal probably begins to take food at this stage. 



On the eighth day a horseshoe-shaped group of nuclei is 

 seen in stained specimens on the back of the head, rather 

 behind the eyes ; the two points of the horseshoe are 

 directed backwards, and the nuclei are arranged in a double 

 line. It is probable that this is the rudiment of the ciliated 

 " olfactory organ" (the "corona ciliata " of Grassi). 



The abdominal ganglion begins to be gradually reduced in 

 size relatively to the rest of the body, and its elements 

 become more completely separated, so that the nuclei are 

 packed in a dense mass at each side of the ganglion, while 

 the raid-ventral region appears clear, and is composed 

 exclusively of fibres. This gives the ganglion a markedly 

 bilateral appearance, especially in stained preparations. 



In the head the muscles have already assumed the arrange- 

 ment of the adult by the sixth day, and the cerebral and 

 vestibular (lateral) ganglia are now conspicuous. The hood 

 is arranged just as in the adult, and a few days later the 

 cuticular rods which support the teeth (" Stutzplatten " of 

 Hertwig) make their appearance, running forwards and 

 inwards from tlie region of the hooks, and ending in front 

 below the cerebral ganglion. No teeth, however, arc as yet 

 present. 



These changes ai*e all practically complete on the tenth or 

 twelfth day, and on a few occasions when I succeeded in 

 keeping the young alive beyond that time no further altera- 



