1873.] 330 [Morse. 
latter. In the early stages of Discina, the reticulated appearance 
produced by the two layers of muscles is particularly noticeable. 
The presence of a ponderous dorsal and ventral plate, so peculia1 
to many Brachiopods, accounts for the extraordinary muscular 
apparatus to control their movements, as well as to move the body 
upon the peduncle, in those forms that are attached. This muscular 
apparatus is unlike anything we find in the worms, though the power- 
ful retractors of many sipunculoid worms, with their broad expanded 
bases, recall similar features in certain muscles of the Brach- 
iopods. ‘The massive character of the muscles is more like the 
Lamellibranchiates, save that in the latter the muscles are transverse, 
and their only function seem to be to close the shell, their relaxation 
allowing the elastic ligament within or without the shell to force or 
pull the shells open, as the case may be. In the Brachiopods no such 
ligaments are seen, the dorsal and ventral plates being opened, as 
well as closed, by special muscles. In one group of Brachiopods the 
plates interlock at their posterior margins, and are restricted to 
opening and closing in a vertical line. In Discina and Lingula the 
plates do not interlock, and their posterior margins are free, the dor- 
sal one lapping some way over the peduncle; it can therefore swing 
freely back and forth, or oscillate from side to side, as observed by 
Carl Semper in ZL. anatina, and by myself in L. pyramuidaia, in its 
_acts of crawling, or burying itself in the sand. (See Plate I., with 
references.) 
Perwisceral Cavity and Circulatory System. 
In the higher worms, the perivisceral cavity is lined by a delicate 
membrane noticed by Rathke, Quatrefages, Claparede, and others. 
Delle Chiaje designated it the tunica peritoneale. 
In some worms having a rudimentary vascular system, according 
to Claparéde, this membrane is ciliated, and prompts the circulation 
of the perivisceral fluid. This fluid in worms appears to be corpuscu- 
lated and nutritive blood. Most worms appear to possess an exten- 
sive vascular system which contains a colored, but not a corpusculated 
fluid ; this is the pseudo-hemal system of authors. 
In Brachiopods, also, the perivisceral cavity is lined by a delicate 
membrane, which in Terebratulina and Rhynchonella is strongly 
ciliated, as I have plainly observed in living individuals. 
