142 Dr. T. Williams on the Mechanism of Aquatic 



with their pointed base terminating in " two foramina, which 

 diverge and pass into the adjoining cell or cells." 



Fig. 8. Side view of casts of marginal cells. This figure exhibits two 

 young cells originating on the outer side of an old one, causing 

 its aperture to be turned up, thereby giving its outward outline a 

 geniculated form. The small cell to the right reposes completely 

 on one of its sides. 



Fig. 9. Side view of two cells, modified owing to the propinquity of the 

 appendage, of which a cross section is represented. 



Fig. 10. Gutta-percha impression, twice the natural size, of the upper sur- 

 face, showing the somewhat circular and subpolygonal form of 

 the cell-apertures. The marks * * indicate the probable situation 

 of the terminations of the vermiform appendage. 



XIII. — On the Mechanism of Aquatic Respiration and on the 

 Structure of the Organs of Breathing in Invertebrate Animals. 

 By Thomas Williams, M.D. Lond,, F.L.S., Physician to the 

 Swansea Infirmary. 



[With a Plate.] 



[Continued from p. 42.] 



Pulmonifera. 



The leaf-like appendages of Phgllodoce lamelligera, which expose 

 to the action of the aerating medium a true chylaqueous fluid, 

 differ very little in intimate structure from the branchial laminae 

 of the Crab, the purpose of which is to distribute for respiration 

 a current of blood, properly so called. But the trachese of Insects 

 have no parallel amongst the respiratory systems of the Inver- 

 tebrata. Compared with the respiratory organs of the water- 

 breathing Articulata, these trachese constitute, in a complete 

 sense, an apparatus invented de novo. Insects, with reference to 

 the relations of this system, cannot therefore be said to be to 

 the water-breathing Annulose and Articulated animals what the 

 Pulmoniferous Gasteropods are to the water-breathing MoUusca. 

 In the latter cases nevertheless, the aquatic and atmospheric 

 systems are strikingly diverse. 



In the instance of the air-respiring Crustaceans no change of 

 structure whatever occurs in the respiratory system. The bran- 

 chiae of the terrestrial Isopoda are precisely the same in every 

 detail of minute structure as those of the aquatic genera. The 

 inference arises at once : these Crustaceans are merely modified 

 water-breathers ! But when an animal is to be formed whose 

 medium of life shall permanentlg be the atmosphere, a design 

 in the construction of the breathing system is adopted in- 

 volving express provisions, which stand at marked variance 

 from every variety of the water-breathing machinery. It is 



