their Organs of Respiration, 



237 



contact with them, and without any effort on the creature's 

 part. 



The Mollusca which have their branchiae entirely exposed 

 belong to two sub-classes, the Pteropoda and Gasteropoda. 

 In some of these, the branchiae are actually blended with the 

 locomotive organs, as in Clio, a member of the former sub- 

 class, whose fin-like expansions are supposed to perform the 

 office, not of progression only, but also of ventilating the 

 blood as it circulates through the fine regular network with 

 which their surfaces are covered. The Glaucus (Jig. 26.) 



affords another example of 

 the same union of functions. 

 This is one of the most 

 remarkable and most beau- 

 tiful of the Gasteropoda. 

 The body glows with a fine 

 caerulean blue colour, which 

 deepens in hue towards the 

 ends of the fringes of its 

 ptero-branchiae ; the centre 

 of the back is of pearly 

 whiteness, bordered with a 

 line of deep blue; and the 

 sides are adorned with an 

 interrupted series of fan- 

 like laciniated gills, by aid 

 of which, as I have said, it swims reversed at the surface of 

 the Mediterranean Sea, in numerous swarms. 



But, generally, the external branchiae are distinct and in- 

 dependent organs. Of the Pteropoda, almost each genus 

 presents them under some new modification in form, or struc- 

 ture, or position, " as Nature in them strove to show variety." 

 Thus, in the Pneumoderma (Jig. 27. ) 9 they are placed nearly 

 on the posterior extremity of the body, which 

 is naked, and resemble two Cs placed back to 

 back in this manner, DC, united by a little 

 transverse bar across the middle, or at each end, 

 the lines being garnished with a number of re- 

 gular prominent leaflets of minute size. In the 

 Hyales (Jig. 28.), again, the branchiae are pec- 

 tinated, and lie concealed in a space between 

 the lobes of the cloak, to which the water gains 

 admission by certain fissures on the sides of 

 the shell ; while, to make, as it were the dissimilarity perfect, 

 they appear, in the genus Cuviera of Rang (Jig. 29. ), in the 

 form of two small equal and symmetrical processes, exsertile 



Glaucus hexapterygius, copied from Cuvier. 



Pneumoderma, 

 copied from Cuvier. 



