Respiration in Invertebrate Animals. 255 



a new point of view. The author believes that the following is 

 the first systematic exposition on record of the anatomy and signi- 

 ficance of the wow-vascular elements of the lamellibranchiate gill. 

 They are classifiable under two heads. Those parts which 

 are placed between (PL VI. fig. 1 g, fig. 2 e, fig. 8 d; PI. VIII. 

 fig. 19 c, c, c) the parallel bars {the intervectal), uniting them into 

 the form of a leaf, constitute a separate order. Those, secondly, 

 stronger, coarser, in some genera very conspicuous, in others 

 very concealed, which separate the lamellae, forming and bound- 

 ing the excurrent interlamellar water-tubes, to which in many 

 species the ova adhere, the basis of the whole gill, the wonder 

 of the whole enginery, the last of the branchial constituents to 

 be described and understood, are really a distinct and unknown 

 class of structures. 



The first class vary the apparent anatomy of the gill more 

 than the second. They cut the fissural spaces (PI. VI. fig. 7 d) 

 between the individual bars, or individual loops, into oval stig- 

 mata (fig. 5 d), elliptical perforations, or lengthened parallelo- 

 grams (PI. VII. fig. 12 d). In the absence of them, as in 

 Thracia (PI. VII. fig. 15), the intervectal water-fissures are 

 continuous from the free margin of the gill to the proximal. In 

 Mytilus (PI. VIII. fig. 17 o, o, o) they appear under the charac- 

 ter of fleshy nodules ; in Cardium (PL VII. fig. 12 c, c, c) 

 they are almost invisible ; in the Veneridse they consist of a 

 flattened bundle of slender threads, running from bar to bar at 

 equal intervals ; in Pholas they assume almost a membranous 

 form (PL VI. fig. 4e), perforated at regular distances by oval holes; 

 they exist only between alternate loops. In a physiological sense, 

 the highest value attaches to these intervectal parts. They de- 

 termine the dimensions of the water-stigmata. If they are 

 small, the water of respiration is very much subdivided ; if large, 

 the lamella is readily traversed by the aerating element. In 

 calculating the quantum of respiration in the Conchifera, two 

 factors demand to be estimated : first, the amount of blood 

 entering the breathing organ ; secondly, the volume of water 

 by which, in a given time, it is capable of being traversed. The 

 latter will depend upon the dimensions of the water-passages. 



The intervectal connective structures have been mistaken for 

 half a century by the best observers for vessels, blood-canals 

 crossing the bars, — deceiving the observers into the idea that 

 each lamella in the lamellibranchiate gill is really composed of 

 a network of blood-vessels. This idea as regards the Acephala in- 

 volves a fundamental error ; it envelopes everything in unresolv- 

 able confusion. The orbit of the branchial circulation cannot be 

 explained. It contradicts the anatomical arrangement conspicuous 

 in other parts. A consistent sentiment cannot be shaped of this 



