Rci^piratiun in Invertebrate Animals. 147 



reunite, large trunks beconung visible, which end in the auricle. 

 The larger trunks are visible to the naked eye; the smallest 

 require all the skill of the microscopist for their examination. 

 The coats of the more visible white trunks are rendered colour- 

 less under the joint agency of dilute muriatic acid and ;ether : 

 neither reagent alone will remove the white colour. The acid 

 will dissolve the lime and leave the fat : the ivther will remove 

 the fat and leave the lime. The fat and the lime are contained 

 in ce//s, and intinuitely mixed. The lime exists in the form of 

 amorphous granules ; it does not cnjstallize even in the rudi- 

 mentary shell contained in the roof, but from the conical masses 

 which adhere into groups in some places, it is evident that a 

 teiulency to crystallize is manifested. 



It seemed to the author a point of extreme interest to deter- 

 mine whether the lime and fat whigh render the outline of these 

 vessels, especially iu the common Black Slug, so conspicuous, 

 were incorporated in the substance of the vascular walls, and in 

 what manner, or whether they were lodged in a tissue occupying 

 the intervals between the vessels. These questions immediately 

 arose. It appeared so utterly without precedent that vessels, 

 destined to aerate the l)lood, the smallest, the most subdivided, 

 the most thin-walled, the most naked of all the vessels of the 

 body, in every other animal, vertebrate and invertebrate, should 

 in the instance of the Limacidte be encumbered with such thick, 

 even inorganic, rigid parietes ! 



The physiologist is embarrassed in attributing to a machinery, 

 by comparison so coarse and clumsy, a function so subtle and 

 refined as that of breathing. But is it not presumptuous to 

 pronounce a verdict of imperfection upon any of the works of 

 Nature ? Is it not possible that faultless skill and matchless 

 adaptation of instrument to purpose, may yet reveal itself beneath 

 the apparent characters of an organ which at present may appear 

 rude and ill-contrived ? It is more probable that Natm'e should 

 be perfect than that her critic should be so. 



A transverse section of one of the larger vessels will render it 

 at once evident that each trunk is lined internally by a smooth 

 non-calcified membrane, and that the lime-particles are deposited 

 only in the substance of the external coats. The presence of this 

 inorganic substance destroys the power of the vessels to contract 

 upon their contents. The blood is circulated therefore through 

 the pulmonary membrane by some other force than that usually 

 due to the elasticity of the vessels. Although the fact is of dif- 

 ficult proof by direct demonstration, it is almost certain that the 

 exterior of each vessel is also lined by a non-calcified membrane. 

 It must be so, because active vibratile cilia unquestionably exist 



10* 



