1895.] Dr Gaskell, The Origin of Vertebrates. 27 



So characteristic is the structure, so different to anything else, 

 that I have no hesitation in saying that the thyroid of Ammocoetes 

 is the same structurally as the thyroid of Scorpio, and therefore 

 in all probability of Thelyphonus and of Eurypterus. It follows 

 consequently that the respiratory chamber in Ammocoetes just as 

 in Scorpio, Eurypterus, Limulus &c. is formed by a series of 

 branchial lamellar appendages, the most anterior of which is known 

 as the operculum and was connected with the genital apparatus 

 as well as with branchiae. 



Whatever the function of the thyroid may be in Scorpio, in 

 Thelyphonus &c, it is of so great importance to the Ammocoetes 

 that it persists until transformation takes place, i.e. until the time of 

 puberty; then when the animal becomes sexually ripe, it disappears, 

 being eaten up by phagocytes, the branchial segments come to- 

 gether in the middle line and nothing remains of the opercular 

 segment, except the remnants of the thyroid, the hyo-branchial 

 part of the segment and the embryonic puzzle whether the Vllth 

 nerve supplies two segments or one. Still throughout the Verte- 

 brate kingdom the modified thyroid remains to show by the 

 serious effects following its removal what an important part in 

 the economy of the body is played by this modified genital gland. 



The history of the motor part of the Vth nerve. 



This nerve through the whole of the vertebrate class belongs 

 to the same series as the Vllth, IXth and Xth, and as we pass 

 downwards to the fishes, we find that just as the facial nerve was 

 the nerve belonging to the hyoid segment, so the motor part of 

 the Vth supplies the muscles of the mandibular segment, and even 

 as low down as Petromyzon it is still possible to speak of a man- 

 dibular segment. When however we pass still lower to the 

 Ammocoetes we find that great changes have taken place at trans- 

 formation in the muscles supplied by the Vth nerve, and that now 

 the Vth nerve supplies a series of structures which are directly 

 comparable with the appendages of Limulus and Eurypterus. 

 These structures are the velar and tentacular appendages of 

 Ammocoetes, and they correspond strictly with the locomotor and 

 tentacular appendages of Eurypterus and so with the locomotor 

 appendages of Limulus. 



The large velar appendage is the least modified, possessing as 

 it does the arthropod tubular muscles, a blood system of lacunar 

 blood spaces, a surface covered with a regular scale-like pattern 

 formed by cuticular nodosities just as in Eurypterus and in 

 Scorpio, and at its base a serrated gnathite edge guarding on each 

 side the place where the old mouth came to the surface, and still 

 acting as a grinder according to Langerhans, or more probably a 



