T Davidson — Wliat is a Brachiopod ? 
151 
two, three, or four lobes clothed witli vibratile cilia; and before 
becoming attacbed swims, or wbirls bead 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-spots in 
the embryo of Thecidium, and States that the animal appears to be in 
some measure sensible to light. The mouth conducts by a narrow 
cesophagus 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. 
Hantle . — Both valves are lined by a delicate membrane termed tbe 
“pallium”or mantle; it secretes the shell, and is generally fringed 
with horny bristles or setas (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, 1 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 surface of the mantle facing the inner 
surface of the valves, corresponding short cylindrical membranous 
projections, or caeca, which insert themselves into the small tubulär 
orifices that traverse the shell (PL VIII. Fig. 10 c). The cascal pro- 
longations do not exist in those genera such as Rhynclionella where 
the shell is deprived of tubulär 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 lacunm. 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 Becherches'sur l’organisation du Manteau ehez les Brachiopodes articules, Caen, 
1864, and to which the reader is referred for further details. 
