DE. T. DAVIDSON OX RECENT BRACHIOPODA. 199 



" Ou the lobe of the mantle which Hues the imperforate valve these vessels are seen 

 converging from the respiratory margin to four trunks, which are much shorter than 

 the corresponding ones in Terehratula : on the opposite mantle-lobe the branchial vessels 

 form only two such trunks. 



" In this profuse distribution of vessels over a plain membranous expansion, we 

 perceive the simplest construction of the water-breathing organ, or Iranchia ; and while 

 it proves the close affinity of the Brachiopoda to the Ascklui;, it presents, at the same 

 time, a beautiful analogy Avith the elementary forms of the air-breathing organ, as it 

 exists, for example, in \h.Q pulmomferous Gasteropods. 



" The muscular system of Orbicula differs iu some respects from that of Terehratula. 

 Eight distinct muscles may be perceived, without including the labial arms. The four 

 thick and strong muscles which form the anterior and posterior pairs above noticed, do 

 not decussate each other, but pass a little obliquely from one valve to the other. On 

 the lower valve they are attached to the margin of the elevation caused by the oval 

 depression noticed on the exterior of the sliell. Some of the fibres of the large anterior 

 pair pass through the chink in the perforated valve and expand into the organ of 

 adhesion. "Within the space included by the above pairs of muscles, there are two 

 slender pairs of muscles which decussate each other. The superior pair take their origin 

 from the anterior part of the strong membrane that circumscribes and protects the 

 viscera below the stomach, and between the insertions of the anterior shell-muscles ; they 

 then ascend, diverge on either side of the alimentary canal, and are inserted into the 

 opposite valve outside the posterior shell-muscles. The inferior pair arise from the sides 

 of the membranous circle, and converge as they pass below the preceding, to be inserted 

 into the perforated valve on the inner side of the posterior shell-muscles. While, there- 

 fore, the larger muscles have the more important office of guarding the animal by closing 

 the shell, the smaller muscles woidd admit the Avater by sliding the margin of one valve 

 over the other ; and they are also calculated to produce a compression of the viscera. 



"The labial processes or hrachla are scarcely more adaj^ted to protrude externally 

 than in Terehratula chilensis, the only parts that are free being the short spiral extre- 

 mities ; but in the more muscular character of their basis or stem they exhibit a close 

 affinity with Linrjula. Considering the arms as a pair, the stems are tlien joined below 

 the mouth, forming on that aspect a transverse, semilunar, fleshy basis, fringed and 

 convex anteriorly. This is attached to the anterior part of the tendinous belt of the 

 viscera. At the sides of tliis basis the arms make a sudden bend upon themselves 

 towards the mouth, above and in front of which the extremities make a sjoiral turn and a 



half. The bent portions are closely adherent to each other, not free, as in Lingula 



The arms in Orhicida are not, however, suj^ported by an internal calcareous process. 

 The muscular basis, when cut into, exhiljits on each side a well-defined cylindrical cavity, 

 which commences near the mesial plane in the transverse part below the mouth, and is 



continued into the spiral extremity But I conclude, nevertheless, tliat the 



canals serve to extend outwards the free spiral extremities, by being forcibly distended 

 with fluid propelled along them. 



"The mouth, a small puckered orifice, is best seen by dissecting away the transverse 



SECOXD SERIES. — ZOOLOGY, VOL. IV. 27 



