180 



with a disk perforated with tubes, each tube having been secreted by 

 a corresponding tentacle-like filament of the mantle. 



Class 5. BRACHIOPODA. 



Animal ; hotly small, not furnished with branchiae but enveloped 

 by a J ringed mantle, of which the lobes are not lateral, but dor- 

 sal and ventral, having a system of vascular branchiae incor- 

 porated icith them, and each secreting a shelly valve ; no head 

 or eyes ; mouth furnished with two extended ciliary processes 

 or arms, which are sometimes free, folded in coils, sometimes 

 supported on a more or less complicated cartilaginous or cal- 

 careous skeleton ; muscular system elaborate, comprising five to 

 siv pairs of adductor, cardinal, pedicle, and capsular muscles. 



Shell ; always bivalve, the valves being ventral and dorsal, uncon- 

 nected by any ligament, but sometimes closely interlocking with 

 curved tooth-like processes, one valve free, the other agglutinated 

 or affixed by a pedicle to foreign substances. 



The Brachiopods are a tribe of very peculiar structure, intermediate in 

 the general features of their organization between the Lamellibranchiate 

 Bivalves and the Cirrhipedes. Like the former, they secrete a bivalve 

 shell, but the valves are not side-valves, connected dorsally by a ligament 

 and opening ventrally. The valves are themselves dorsal and ventral, 

 unconnected by any ligament, and the mantle-lobes which secrete them 

 perform also the offices of respiration. The branchise are incorporated 

 with the mantle-lobes in the form of a system of vessels, hence the term 

 Palliobranchiata, or mantle-breathing, sometimes given to these mollusks, 

 a much more appropriate term, as we shall presently see, than the almost 

 universally adopted one of Brachiopoda. Projecting from the mouth of 

 these creatures, one on each side, corresponding with the labial palps of 

 the lamellibranchiate bivalves, are two extended, cirrhated, tubular pro- 

 cesses, which are sometimes folded into spiral coils, sometimes attached to 

 and supported by a cartilaginous or calcareous skeleton termed the apophy- 

 sis. These cirrhated processes, called, through some fanciful analogy, arms, 

 were supposed originally to be exserted in constant activity, like the cirrhi 

 or cirrhated arms of the Cirrhipedes, with which they seem to have some 

 affinity; but the arms of the Brachiopod are not feet, in the sense in which 

 the cirrhi are feet in the Cirrhipedes, nor are they feet in the sense in 

 which the belly is a foot as an organ of locomotion in the Gasteropod. 



