﻿T. Davidson — What is a Bmchiopod ? 153 



stretclied out, they are said to be more than four times the leugth of 

 the shell, and to support some 3000 cirri (PL IX. Fig. 8). It 

 must, however, remain for ever uncertain whether in the extinct 

 genera Spirifera, Atrypa, and others, in which the spirally coiled 

 fleshy arms were supported throughout their entire length by a 

 calcified skeleton, the animal could protrude its labial appendages 

 beyond the margin of the valves. 



In some families — Bhynclionellidce, Procluctidce, 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 they 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 origin), 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 septenti-ionalis. A cluster of eggs from the genital band (magnified). 



2. ■ Egg from the perivisceral chamber, immediately after their escape 



from the pallial sinuses. 



3. ^oa''> 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 themselves by their caudal segment. 



1 1. Embryo showing first appearance of caudal or peduncular segment. 



12, 13, 14. Successive changes of the embryo, showing formation of dorsal 



and ventral areas by the folding and growth of the thoracic ring. 



15, 16. Embryo, the deciduous setse appear in this stage. 



17. 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 setse, and contour 



of embryonic shell. 



20. A considerably advanced stage, showing subsequent widening of 



the anterior portion of the dorsal and ventral plates. 

 All the above erdaged figures are taken from Prof. Morse's memoir on the 

 embryology of Terebratulina. 



21. Waldheimia cranium. Showing the manner in which it attaches itself by the 



means of its peduncle, a. ventral ; b. dorsal valve ; p. peduncle. 



22. Discina lamellosa. Showing the manner in which it attaches 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. peduncle. 



23. Crania ignabergensis (fossil). Showing the manner in whicb it attaches 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 sphceroidalis (fossil). Longitudinal section to show the loop and 



large free space for the occupation of the animal, a. ventral ; b. dorsal 

 valve. 



