THE PROSOMATIC SEGMENTS OF AMMOCCETES 32 1 



arrangement of the tubes. In Fig. 126, A, I give a picture of a piece 

 of the coxal gland of Limulus taken from Lankester's paper. 



Turning now to the vertebrate, Bela Haller's paper gives us a 

 number of pictures of the glandular hypophysis from various verte- 

 brates, and he especially points out the tubular nature of the gland 

 and its solidification in the course of development in some cases. 

 In Fig. 126, B, I give his picture of the gland in Ammoccetes. 



The striking likeness between Haller's picture and Lankester's 

 picture is apparent on the face of it, and shows clearly that the 

 histological structure of the glands in the two cases confirms the 

 deductions drawn from their anatomical and morphological positions. 



n a 



Fig. 126. — A, Section of Coxal Gland op Limulus (from Lankester) ; B, 

 Section of Pituitary Body of Ammoccetes (from Bela Haller\ 



n.a., termination of nasal passage. 



The sequence of events which gave rise to the pituitary body 

 of the vertebrate was in all probability somewhat as follows : — 



Starting with the excretory glands of the Phyllopoda, known as 

 shell-glands, which existed almost certainly in the phyllopod Trilo- 

 bite, we pass to the coxal gland of the Merostomata. Judging from 

 Limulus, these were coextensive with the coxse of the 2nd, 3rd, 4th, 

 and 5th locomotor appendages. When these appendages became 

 reduced in size and purely tactile they were compressed and con- 

 centrated round the mouth region, forming the endognaths of the 

 Merostomata ; as a necessary consequence of the concentration of the 

 coxse of the endognaths, the coxal gland also became concentrated, 



