162 AETHUK WILLEY. 



is to be noted tliat while the primitive rehitions of each couple 

 of stigmata are indicated by the slight curvature of their 

 ventral ends, yet the actual order of their formation could 

 not be deduced from their appearance and size. For example, 

 D appears larger than B, from which it in reality was derived. 



A later condition of the protostigmata is shown clearly in 

 the specimen represented in fig. 12. Here we still see the 

 long-drawn-out dorso-ventral clefts, but the first one has 

 undergone subdivision into four approximately equal secon- 

 dary branchial stigmata. The method of division by the 

 growth of a tongue-like projection across the primitive cleft 

 is shown in fig. 13. 



The vascular trabeculee, which traverse the wall of the 

 pharynx in the longitudinal direction, pass across the proto- 

 stigmata independently of the tongue-like bridges which 

 subdivide them. In the specimen represented in fig. 12, only 

 the first protostigma is subdivided, but there were four 

 longitudinal vascular trabeculffi passing from the buccal region 

 to the fundus of the pharynx, crossing over the stigmata, and, 

 in the case of the first stigma, not corresponding with the 

 true subdivisions of the cleft. They have been omitted from 

 the lithographed drawing for the sake of clearness. 



It should further be noted that at the time when the first 

 protostigma is subdivided, the sixth is still quite small. I 

 am unable to say whether any fresh perforation occurs after 

 the formation of the series of protostigmata, but certainly the 

 bulk of the secondary branchial stigmata arise by subdivision 

 of and abstriction from the protostigmata. 



The transverse rows of secondary stigmata do not long 

 retain the simple form which they present in fig. 12. They 

 soon become arched in the manner shown in fig. 14, where it 

 is seen that the concavities of the arches face one another. 

 Next occurs the abstriction of small tertiary stigmata, which 

 take part in the formation of the intercalary rows (fig. 15). 

 Finally, by differential growth, the stigmata gradually assume 

 the spiral disposition which is so characteristic of the Molgu- 

 loid pharynx. 



