202 ARTHTJE B. SHIPLEr. 



is afterwards increased by a pair of new ones arising between 

 each of those already formed. The tentacles subsequently 

 become branched (fig. 39). 



With regard to the mesoblast of the head I have little to 

 add to the descriptions of Balfour and Scott. The area 

 over which the gills extend at their first appearance extends to 

 the posterior boundary of the sixth myomere. The most 

 anterior myomere is situated close behind the ear, and the ear 

 lies above the hyobranchial or first persistent gill-cleft. So 

 that at their first appearance the six posterior gill-clefts cor- 

 respond in their extent with the six anterior myomeres. As 

 the larva grows the gill region appears to elongate with rela- 

 tion to the muscular myomeres, so in my latest larva there are 

 about nine myomeres over the area of the six gills (fig. 43). 

 These anterior myomeres become V-shaped with the open 

 angles directed forwards ; turned the opposite way to those of 

 Amphioxus. 



The mesoblast between the gills arranges itself into head 

 cavities (fig. 21), and as Balfour and Scott have already 

 shown, there are two head cavities in front of the hyomandi- 

 bular cleft. These are at first continuous, but with the for- 

 mation of the stomodseum they separate. One becomes prse- 

 oral and obviously corresponding with the prse mandibular 

 head cavity of Elasmobranchs; the other with the mandi- 

 bular (fig. 21). The walls of these cavities ultimately form 

 the skeleton of the gill arches", the muscles of which are all 

 of the tubular kind. Owing to the rudimentary condition of 

 the eye in Ammocoetes, no eye-muscles are present and conse- 

 quently it is impossible to say whether or no they are derived 

 from the walls of the head cavities, but the researches of 

 Stannius and Langerhans have shown that they possess 

 the same histological characters as the muscles of the gills and 

 upper lip. 



The Central Nervous System, 

 The development of the central nervous system has been 

 described above up to the stage when the central canal has 



