﻿T Davidson — What is a Bmchiopocl? 151 



two, tlii-ee, or four lobes clothed with vibratile cilia; and before 

 becoming attached swims, or whirls head foremost, by means of the 

 vibratile cilia covering the body. Morse describes the gradual for- 

 mation of the shell from its first stage of development to the adult 

 condition. Lacaze-Duthiers also alludes to two and four eye-sjDots in 

 the embryo of Thecidium, and states that the animal appears to be iu 

 some measure sensible to light. The mouth conducts by a narrow 

 oesophagus to a simple stomach, which is surrounded by a large 

 granulated liver. Owen's " hearts " have been found to be oviducts, 

 while the true heart would, according to Huxley and Hancock, 

 consist of a pyriform vescicle appended to the dorsal surface of the 

 stomach ; but Prof. Semper, who has described the animal of Lingula 

 at considerable length, with especial reference to its vascular system, 

 contends that the pyriform vescicle termed " a heart " does not repre- 

 sent that organ, that there is not the least vestige of vascular system 

 connected with it, and consequently that the existence of a heart must 

 be considered unproven. The digestive organs, viscera, as well as 

 the muscles, which take up only a small place in the proximity of 

 the beak, are separated from the great anterior cavity, and protected 

 by a strong membrane, in the centre of which the mouth is situated. 

 The nervous system consists of a principal ganglion, of no great size. 



Mantle. — Both valves are lined by a delicate membrane termed the 

 "pallium" or mantle; it secretes the shell, and is generally fringed 

 with horny bristles or setaa (PL VIII. Figs. 9 and 10). It is composed 

 of an outer and inner layer, between which are situated the blood- 

 channels or lacunes. The mantle has been ably described by Hancock, 

 and by E. Deslongchamps,' who observes " that all the internal parts 

 of the shell are lined by the internal layer of the mantle, with the 

 exception of the muscular impressions, or those portions where the 

 muscles are inserted on the inner surface of the shell." 



The outer layer closely lines the inner surface of the valves to 

 which it adheres, and in those species in which the shell is traversed 

 by canals there exists on the sui-face of the mantle facing the inner 

 surface of the valves, corresponding short cylindrical membranous 

 projections, or cseca, which insert themselves into the small tubular 

 orifices that traverse the shell (PI. VIII. Fig. 10 c). The c^cal pro- 

 longations do not exist in those genera such as Rliynchonella where 

 the shell is deprived of tubular perforations. The inner layer is 

 rather thicker than the outer one, and is covered with vibratile 

 cilia. Between the two layers composing the mantle are situated 

 the blood-channels or lacunaa. These vary in their dispositions or 

 details in different genera, and as they project to some small extent, 

 leave corresponding indentations on the inner surface of the shell, 

 so that their shape and directions can, very often, be traced on fossil 

 and extinct genera, as well as if the animal were still living, as may 

 be seen in the numerous illustrations appended to my works on 

 British and Foreign Brachiopoda. 



There are usually four principal arterial trunks in each lobe of the 



1 Eec]ierclies>ur 1' organisation du Manteau chez les Brachiopodes articule^, Caen, 

 1864, and to wliich tlie reader is referred for further details. 



