Respiration in Invertebrate Animals. 143 



accordingly observed that the contrast between ' the lung ' of 

 Pulmoniferous, and the branchia of the Branchiferous Gaste- 

 ropods is almost as striking and irreconcileable as that which 

 separates the trachea of the Insect from the breathing plume 

 of the Annelid or the gill of the Crab. In the midst of the 

 aquatic Articulata, the air-breathing Insect arises on the scene ; 

 in the midst of the aquatic Mollusca, the pulmonated Gas- 

 teropods are formed. The object being one and the same, 

 namely to produce an air-breathing animal, the artificer being 

 still within the liaiits of the Invertebrate subkingdom, the ques- 

 tion is most natural, Are the means in the two cases also the 

 same by which the one and the same end is sought to be attained ? 

 — No ! they are most wonderfully and extraordinarily dissimilar. 

 The tracheae of the Insect pervade the entire substance of the 

 body of the animal. The ^ lung ' of the Snail is a mere bag, in- 

 flated as if by some rude and fallible artist, under the skin of 

 the back. The former charms the eye as it discovers the mingling 

 of the infinitely perfect with the infinitely minute. The latter 

 shocks the mind with disappointment as it views the characters 

 of a contrivance at once coarse, clumsy, and inadequate. But is it 

 so in reality, or is it so only because imperfectly understood, and 

 because it is measured by a wrong and unfair standard ? Is 

 not such an apparatus, simple as it is, quite enough to sustain 

 the sluggish vitality of these slow-moving and sleepy animals ? 

 And is not the end in view accomplished quite as perfectly as it 

 is in the case of Insects, though by a machinery of incomparably 

 greater apparent intricacy ? These questions will well prepare 

 the mind for the investigation of the actual details. 



All the terrestrial and the majority of" the freshwater Gaste- 

 ropod Mollusks breathe air. They are provided with a pul- 

 monary cavity or sac, whose walls are networked with vessels by 

 which the blood is exposed to the aerating element. No form 

 of branchiae exists. The animals which present this organization 

 are all provided with distinct heads and furnished with tentacula 

 and organs of sight. They walk by the aid of a well-developed 

 creeping disc. One large division of the land snails is supplied 

 with an operculated shell ; the rest are inoperculate and some- 

 times shell-less. The Pulmonifera are closely related to the 

 plant-eating sea snails (Holostomata) through the Cyclomata, 

 and to the Nudibranchs by Onchidium. As a group, the land 

 snails are inferior to the sea snails, on account of the compara- 

 tive imperfection of their senses, and the union of the functions 

 of both sexes in each individual. 



The typical inoperculate Pulmonifera vary in appearance 

 and habits, but agree essentially in structure. The respiratory 

 orifice is small and valve- like, to prevent too rapid desiccation 



