T. Davidson — What is a Brachiopod ? 
153 
stretclied out, tliey are said to be move thau four times tlie length of 
the sbell, and to support. some 3000 cirri (PI. IX. Fig. 8). It 
must, however, remain for ever uncertain wkether in the extinct 
genera Spirifera, Atrypa, and others, in which the spirally coiled 
fleshy arms were supported tkroughout their entire length by a 
calcified skeleton, the animal could protrude its labial appendages 
beyond the margin of the valves. 
In some families — Rhynchonellidce, Productidoe, and others — these 
Organs are spiral and separate ; in others, Lingulidce, only at their 
extremities. It is almost certain that these beautiful organs, by 
means of their cirri and the cilia tkey are doubtless furnished with, 
are not only instrumental in carrying floating nutrimental particles 
or minute microscopic organisms to the mouth (which is situated 
between the appendages at their oi'igin), but are subservient to the 
function of respiration. Hancock observes that “ to prove that the 
brachial organs subserve the function of gills, as well as that of 
sustentation, it is only necessary to refer to the manner in which the 
blood circles round the arms (labial appendages), and is carried to 
the cirri ; but more particularly to its circulating through these latter 
organs, returning direct from them to the heart.” 
{To be continued in our next Number.) 
fig. EXPLANATION OF PLATE VII. 
1. Terebratula septentrionalis. A cluster of eggs from the genital band (magnified). 
2. - Egg from the perivisceral chamber, immediately after their escape 
from the pallial sinuses. 
3. Egg ; first ciliated stage. 
4. 5. — Transverse division of embryo, showing long tuft of cilia at cephalic 
extremity. 
6, 7, 8. Embryo, showing various outlines assumed in, while swimming. 
9, 10. Embryos attaching tliemselves by their caudal segment. 
1 1 . Embryo showing first appearance of caudal or pedimcular segment. 
12, 13, 14. Snccessive changes of the embryo, showing formation of dorsal 
and ventral areas by the folding and growtb of the thoracic ring. 
15, 16. Embryo, the deciduous setae appear in this stage. 
1 7. Free swimming embryo. 
18. First stage, in which the mouth makes its appearance, and dorsal 
and ventral plates become distinctly marked. 
19. Earliest stage, showing arrangement of deciduous setae, and contour 
of embryonic shell. 
20. A considerably advanced stage, showing suhsequent widening of 
the anterior portion of the dorsal and ventral plates. 
All the above enlaged figures are taken from Prof. Morse’s memoir on the 
embryology of Terebratulina. 
21. Waldheimia cranium. Showing the manner in which it attaehes itself by the 
means of its pedunele. a. ventral; b. dorsal valve; p. pedimcle. 
22. Discina lamellosa. Showing the manner in which it attaehes itself and forms 
clusters of large dimensions. Upwards of one hundred specimens thus 
attached were sent to me by Prof. Verrill, from Callao, Peru. p. pedunele. 
23. Crania ignabergensis (fossil). Showing the manner in whicli it attaehes itself to 
submarine rocks and corals by a portion of the surface of one of its valves. 
24. Chonetes striatella (fossil). To show the row of hollow spines that rise from the 
upper edge of the area of the dorsal valve. 
25. Terebratula sphmroidalis (fossil). Longitudinal section to show the loop and 
large free space for the occupation of the animal, a. ventral ; b. dorsal 
valve. 
