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 into veins ; 

 the arterial blood flows, according to the interesting discovery of 

 MILNE EDWARDS 1 , not through closed vessels, but in reticular inter- 

 spaces (lac-unce), which are emptied into larger venous sinuses, [or 

 rather the so-called close vessels expand, their attenuated proper 

 tunic being continued into these lacunae and sinuses 2 ]. 



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. Each 

 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 Area, 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 3 . The gills in this condition resemble those of bony 

 fishes, whilst the laminated structure, which in the Lamettibran- 

 chiata is that generally prevalent, occurs in these fishes (in Xipliias 

 gladius) only as the exception. Another deviation from the ordi- 

 nary type of the gills is seen in some genera of Lamettibranchiata 

 in the number of these organs, when on each side, instead of two, 

 only a single gill is present 4 . From the common opinion that the 



1 Ann. des Sc. Nat. 3ieroe Se'rie, in. 1845, pp. 300 304. 



2 [Vid. OWEN Introduction to the Anatomy of Terebratula in DAVIDSON'S Mono- 

 graph on Fossil Terebratulce, published by the Palteontographical Society, 1853, pp. 

 15, 1 6, and PI. in. fig. i. 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, 1.8.] 



3 MECKEL'S System der vergl. Anat. vi. 1833, p. 60. In Solenomya the gills also are 

 feather-shaped; seePniLiPPi in WiEGMAXx'sArchivfiirNaturgeschichte, 1. 1835, B - 2 75- 



4 In Anatina and Pholadomya SOWEHBY the gill-plates on each side are so grown 

 together as to form a single gill ; OWEN Lectures on the Comp. Anat. of the invert. 



