Respiration in Invertebrate Animals. 29 



serve to establish the distinctness of species quite as clearly and 

 convincingly as the grosser characters of the outward append- 

 ages. The branchial plates of two separate species, in general 

 position, in form and size, &c., may to the casual eye of 

 the descriptive naturalist, appear absolutely identical. Guided 

 by the microscope, the minute anatomist, however, detects 

 organic dissimilarities which enable him at once to assign the 

 objects under view to two distinct animals. A thousand illus- 

 trations of this kind may be readily adduced to prove the im- 

 portance of minute investigations of structure. False analogies 

 suggested by general external resemblances of organs can be 

 authoritatively corrected only by an appeal to the facts of ulti- 

 mate structure. How utterly confused, how deeply deficient are 

 the views of the comparative physiologists even of these advanced 

 times, as to the history of the renal and urinary systems of the 

 Invertebrate animals ! How difficult, in any given instance of 

 doubt, to state whether an organ is a kidney or not ! The ulti- 

 mate elements, those last factors which constitute the essence 

 of the organ, are utterly unknown. For it is not even now 

 determined what is and what is not essential to the kidney of an 

 Invertebrate animal. This opprobrium applied but a short time 

 since to the fluid systems and to the respiratory organs. How 

 great are the honours yet in store in this field of research for 

 the clear thinker and fruitful observer ! 



The general affinities of the Pectinibranchiate order of Mol- 

 lusks are familiar to all. In all the genera, a spacious branchial 

 chamber exists (PI. V. figs. I & 2). It is a recess over-vaulted by 

 the anterior termination of the mantle. It occupies the last turn 

 of the spire. It is open in front. This arrangement will be after- 

 wards contrasted with the closed character which it exhibits in the 

 Pulmonifera. The Pectinibranchs admit of division into two sub- 

 orders — the Holostomata and the Siphonostomata. In the former, 

 the margin of the shell and mantle is entire ; in the latter, it is 

 either notched or produced into a canal or siphon (/). Through 

 this prolongation of the mantle the water enters the chamber. 

 In the Holostomata it penetrates at the same point in the 

 fissure between the dorsum of the animal and the edge of the 

 mantle. The machinery by which the ingress and egress of the 

 water are efi'ected resides in the branchial hood of the mantle. It 

 performs regular respiratory movements. These movements, 

 however, are aided by the invisible agency of cilia. The Pectini- 

 branchs are prosobranchiate. The heart in all affects a position 

 immediately behind the branchial organ. The aerated blood 

 returning from the latter is received directly by the auricle of 

 the former. 



What is designated the branchial vault in the Pectinibranchs 



