220 TOWARDS THE HUMAN FORM 



region, which is deprived of all contact with the exterior 

 environment. Hence their heads developed those voluminous 

 plumes covered with vibratile cilia which by their movements 

 draw towards the organisms both food particles and water 

 charged with oxygenated air. On account of these plumes, 

 Lamarck called all these worms Cephalobranchs. 



Brachiopods enclosed between the two valves of their shell 

 obtain food in exactly the same way. Their mouth lies between 

 two respiratory plumes that are either twisted spirally 1 or 

 like a screw, 2 or variously convoluted before being rolled. It is 

 to these plumes that the Brachiopods, now under consideration, 

 owe their name. In studying the embryogeny of the Lingular, 

 which existed already in the Cambrian Epoch, and have 

 scarcely been modified since that distant period, Morse was 

 struck by the resemblance between these young organisms and 

 Serp-ula. The two valves of the shell develop in a region of 

 the organism's body corresponding to the collar of Serpula. He 

 was thus led to consider the Brachiopods as cephalized Annelids. 

 Embryogeny, moreover, leaves no doubt but that these 

 organisms are derived from Annelid Worms with a reduced 

 number of segments. Their resemblance to bivalve Molluscs 

 is entirely superficial. The valves of their shell are dorsal and 

 ventral, instead of left and right, like those of the Molluscs, 

 and their texture is totally different. Their internal structure 

 has nothing in common with that of Molluscs. On the other 

 hand, the Brachiopod clearly approximates in internal structure 

 to one or two segments (Rhynchonella) of the Annelid Worm 

 that have become individualized. 



This said, we can distinguish two main types of Brachiopods : 

 Firstly, the Inarticulata, in which the valves of the shell, 

 with their somewhat horny consistency, are independent of 

 each other; secondly, the Articidata, in which the two strongly 

 calcified valves are united with each other by a real hinge. 

 It is muscles that open the shell, and not a mere ligament 

 which springs back as in the bivalvular Molluscs or Lamelli- 

 branchs. Furthermore, the shell of a dead Brachiopod is 

 obstinately closed, whereas that of a Lamellibranch gapes. 

 The Lingular live in the sand, where they bury their long, 

 mobile peduncle, which represents, according to Morse, the 



1 Terebratulidae. 2 Rhynchcnellidae such as Spiri/er, etc. 



