192 



THE ORIGIN OF VERTEBRATES 



from the internal surface, might be represented by Fig. 79, in which 

 the thick line represents the outline of the opercular segment, and 



the fainter lines the succeeding 

 branchial segments. The middle 

 and anterior part of the opercular 

 segment carried the terminations 



Gen. dncfc. 



TJt. M&sc. 



Gen. duct. 



Fig. 79. — Diagram to indicate the 



PROBABLE NATURE OF THE MeSOSO- 



matic Segments of Eurypterus. 



The opercular segment is marked out by 

 the thick black line. The segments 

 II. -VI. bear branchiae, and segment I. 

 is supposed in the male to carry the 

 uterus masculinus (TJt. Masc.) and 

 the genital ducts. 



of the ' genital 



organs 



these I 



have represented, in accordance 

 with our knowledge of the nature 

 of these organs in the present-day 

 scorpions, as a median elongated 

 uterus, bilaterally formed, from 

 which the genital ducts passed, 

 probably as in Limulus, towards 

 a mass of generative gland in the 

 cephalic region, and not as in 

 Scorpio or Thelyphonus, tailwards 

 to the abdominal region. 



It is possible that in Holm's 

 representation of Eurypterus, Fig. 

 104, the genital duct on each side 

 is indicated. 



The Thyroid Gland of Ammoccetes. 



If we compare this mesosomatic region of Eurypterus with that 

 of Ammoccetes, the resemblance is most striking, and gives a mean- 

 ing to the facial nerve which is in absolute accordance with the 

 interpretation already given of the glossopharyngeal and vagus 

 nerves. In both cases the foremost respiratory or mesosomatic 

 segment is double, the posterior lateral part alone bearing the 

 branchiae, while the median and anterior part bore in the one animal 

 the uterus and genital ducts, in the other the thyroid gland and 

 ciliated grooves. We are driven, therefore, to the conclusion that 

 this extraordinary and unique organ, the so-called thyroid gland of 

 Ammoccetes, which exists only in the larval condition and is got rid 

 of as soon as the adult sexual organs are formed, shows the very form 

 and position of the uterus of this invertebrate ancestor of Ammo- 

 ccetes. What, then, is the nature of the thyroid gland in Ammoccetes ? 



