710 CLASS XII. 
heart to the gills. When a single heart is present, it is usually 
seated between the two membranous triangular auricles that have 
their points turned towards the heart. From the heart arise the 
arteries, which, however, do not pass from capillaries mto veins; 
the arterial blood flows, according to the interesting discovery of 
Mine Epwarps}, not through closed vessels, but in reticular inter- 
spaces (/acune), which are emptied into larger venous sinuses, [or 
rather the so-called close vessels expand, their attenuated proper 
tunic being continued into these lacunz and sinuses?]. 
The respiratory organs are, in the Brachiopoda, situated on the 
mantle, or the vascular mantle itself serves for respiration. In the 
Lamellibranchiata the gills lie as plates between the margins of 
the mantle. Ordinarily two gills are present on each side. Hach 
gill consists of two plates, which are more remote from each other 
on the dorsal surface, and coalescent at their free outer margin. 
Sometimes the triangular spaces which are thus formed in the gills 
are capable of a great extension, and serve, as in Anodonta, as tem- 
porary repositories for the eggs,—brooding cavities. On each plate 
numerous transverse stripes or projecting lines are seen, along 
which the currents of blood pass in the gills. In Arca, Pecten, and 
Spondylus, each of these projecting lines is changed into a free fila- 
ment, and the gills thus consist here not of plates but of threads, 
which, though separate, are still from their numbers placed close 
together®. The gills in this condition resemble those of bony 
fishes, whilst the laminated structure, which in the Lamellibran- 
chiata is that generally prevalent, occurs in these fishes (in X¢phias 
gladius) only as the exception. Another deviation from the ordi- 
nary type of the gills is seen in some genera of Lamellibranchiata 
in the number of these organs, when on each side, instead of two, 
only a single gill is present*. I*rom the common opinion that the 
1 Ann. des Sc. Nat. 3ieme Série, 11. 1845, pp. 300—304. 
2 [Vid. OwENn Introduction to the Anatomy of Terebratula in Davipson’s Mono- 
graph on Fossil Terebratule, published by the Palaeontographical Society, 1853, pp. 
15, 16, and Pl. 11. fig. 1. This structure of the spaces in which the blood flows was 
first explained by Hunter, and exists equally in insects and crustaceans. OWEN 1. 1. 
pp- 17, 18.] 
3 MeEcKEL’s System der vergl. Anat. V1. 1833, p.60. In Solenomya the gills also are 
feather-shaped ; see PHILIPPI in WIEGMANN’S Archiv fiir Naturgeschichte, 1. 1835, 8. 275. 
4 In Anatina and Pholadomya SowERBY the gill-plates on each side are so grown 
together as to form a single gill; Own Lectures on the Comp. Anat. of the invert. 
