OF THE BRACHIOPODA. 



263 



lining of the perivisceral space, as well as the investing ovarian membranes, but the details of the 

 very rudimentary circulatory system of the Brachiopoda are imperfectly known. Some of the 

 Brachiopoda were formerly credited with the possession of two or even four pulsating organs. But 

 the so-called " hearts " of Cuvier and Owen are now known to be connected with the oviducts ; while 

 the " pyriform vesicle " of Hancock and Huxley has proved 

 to be non-contractile, without muscular walls, and to have no 

 connection with the arterial system. Semper has demonstrated 

 the non-existence of a heart in Lingula, and Morse has more 

 recently maintained that the circulation of the blood is entirely 

 due to " ciliary action." Thus the presence of the pliant, hair- 

 like cilia, so often referred to as present on the mantle, the 

 lining of the body cavity, the cirri of the arms, and in the tube 

 at their base, acquires additional significance, for by their exclu- 

 sive agency the blood is distributed through the complicated 

 vascular system ; while the function of aeration, generally effected 

 by the gills the equivalents of the lungs of other animals is 

 among these lowly molluscs, divided between the mantle and the 

 " arms," a fact that illustrates the imperfections of the Brachio- 

 podal organisation, and their lowly rank in the molluscan type. 



The nervous system is composed of two nerve-centres, united 

 by a nerve, forming a collar round the gullet, and communicating 

 by a series of minute fibres with the arms, the mantle, and 

 pseudo hearts the oviducts. Being headless, or acephalous, 

 the Brachiopoda have no brain. But Professor Morse has 

 detected the presence of a rudimentary organ of hearing in 

 Lingula. Eye-spots are developed in all the embryos, but dis- 

 appear in the later stages of growth. Yet the adult animals are 

 sensitive to light ; and although possessing neither well-developed 

 foot, head, eyes, heart, nor one localised breathing organ, ex- 

 hibit a certain amount of activity and intelligence in fulfilling all 

 the necessary purposes of their existence. The following description has been given by Professor 

 Morse of the mode of life of a youthful individual of the genus Terebratulina : " The animal whirled 

 quickly on its peduncle. When at rest the valves were always closed, and rested on the rock. From 

 this position it turned slowly more than half-way round, raising the body at the same time almost 

 erect. This movement being completed, the valves would very slowly open, and the cirri expand as if 

 to perform a grasping motion ; in no case, however, were they projected beyond the margins of the 

 valves. The cilia lining the cirri produced gentle currents in the water. In this position, with the 

 valves widely open and cirri expanded, the animal would remain motionless for twenty or thirty 

 seconds, and then, with an abrupt closing of the valves, suddenly assume its first position. In 

 watching these motions for a long time, one could not help being impressed with the fact that caution 

 was evidently indicated in the slow and careful movements made in elevating and opening the shell ', 

 while the prompt closing of the valves, and the alert manner in which the animal regained its first 

 position, seemed to show that food had been secured, and further caution was unnecessary." 



Nearly all the Brachiopoda can be referred to one or other of the two great groups into which 

 the class has with common consent been divided, a separation based upon the important structural 

 differences already detailed, which are epitomised in the names Tretenterata and Clistenterata 

 respectively, applied by Prof. William King. The primary groups are again subdivided into eleven 

 families or assemblages of allied forms, each of which may contain either one or several genera and 

 sub-genera. In accordance with the system elaborated by Mr. Thomas Davidson, in his numerous 

 memoirs on British and Foreign Fossil Brachiopoda, these minor classificatory relationships are 

 connected with the chief structural peculiarities of the animal the fleshy breathing organs their 

 nature and shape differing considerably in those forms in which the internal skeleton is absent, or only 

 slightly developed, and depending upon its variable modifications when present. The supporting 



Fig. 8. ANIMAL OF LINGfLA ANATIXA, 



ENLAUGED. (After Hancock.) 



a, Dorfal Lobe of Mantle; v. Vascular SyfUra: h, 

 Brnchial Organs ; ?, Cirri ; /, Perivicceral Cavity : 

 r, Palli:tl Chamber ; e. Intestine ; g, Amis ; r, Liver ; 

 I, Lateral Muscles. 



