Characteristics of Brachiopoda. xcix 



ordinarily the case with animals which are destitute of the power of 

 moving- from place to place, and thus accomplishing* sexual congress, 

 they are found frequently placed closely together. They are always 

 marine. They fall into two great Sub-classes, accordingly as they 

 possess, or as they are destitute of an anus. The proctuchous 

 Sub-class is represented hjLingnlidae, Discinidae, and Craniadae ; the 

 aproctous by Rhynchonellidae and TerehraiuUdae. The proctuchous 

 Brachiopoda differ from the aproctous in having no hinge to their 

 shell, in having a vasiform instead of a globular heart ; in having a 

 convoluted intestine instead of one describing but a simple curve ; 

 in the much smaller evolution of their nervous system, the existence 

 of which has not, as yet, been fully demonstrated ; and in the 

 limitation of their generative glands to their perivisceral chamber. 

 In the existing species of Brachiopoda provided with a hinge, a 

 calcareous process of greater or less length and of various shapes is 

 given off from it for the support of the arms. Some fossil arti- 

 culate Brachiopoda were destitute of these calcified supports, and 

 they are absent in all the hingeless Sub-class. The mouth opens as 

 a simple unarmed transverse slit between the two arms; and it is 

 by the action of the cilia covering their cirri that the ingestion of 

 food, as also the aeration of the blood, is effected. The mantle cavity, 

 a very large part of which is occupied in all Brachiopods, and 

 especially in the aproctous Sub-class, by the arms, is continuous 

 through the oviducts or ' pseudo-hearts,'' with a multiramified sys- 

 tem of interviscerally-placed cavities and canals, which make up 

 the perivisceral system, and in Terehrafiilidae are prolonged into 

 the arms. The ' pseudo-hearts,^ or oviducts, consist each of them of 

 two segments — the one which opens externally being tubular, and 

 the one which brings the exterior communication with the peri- 

 visceral cavities being of wider calibre. They appear to correspond 

 with the organs of Bojanus in the Lamellibranchiata. They give 

 passage outwards to the products of the generative glands. 



A nerve-system has been demonstrated in the hinged Brachiopoda, 

 and consists of five ganglia, connected so as to form a collar around 

 the commencement of the oesophagus. Three of these ganglia are 

 placed below the oesophagus, and the other two at the base of the 

 arms. 



9 2 



