PHYSIOLOGY OF THE INVERTEBRATA, 201 



inner surface of the cavity and the outer surface of the 

 intestine are covered. This movement, which extends into 

 the tube of the common stock, is equivalent to a true circu- 

 lation of the blood. Consequently, the function of circulation 

 in these animals is comparable to that in the Ocelenterata, 

 for the blood is not, during any part of its course, contained 

 in any system of vessels, but is free in the body cavity. 



The Beachiopoda. 



The sinuses met with in the Brachiopoda are the result of 

 partial limitation of the general cavity of the body, for a 

 special purpose. These sinuses "extend into each lobe of 

 the mantle, and end ceecally at its margins. The lobes of 

 the mantle are probably, together with the ciliated tentacula, 

 the seat of the respiratory function. The sinuses of the 

 pallial lobes of Lingula give rise to numerous highly con- 

 tractile, teat-like processes, or ampullae. During life the 

 circulating fluid can be seen rapidly coursing into and out 

 of each ampulla in turn." 



Between the ectoderm and the lining membrane qf the 

 sinus-like "prolongations of the perivisceral cavity in the 

 mantle, and between the endoderm, the ectoderm, and the 

 lining membrane of the perivisceral cavity itself, there is an 

 interspace, broken up into many anastomosing canals," which 

 Prof. Huxley considers, " to represent a large part' of the 

 proper blood systems." Dilatations of these canals have been 

 erroneously described as hearts, biit they are not contractile. 

 " Although the existence of a direct communication between 

 the perivisceral chamber and the blood canals has not been 

 demonstrated, it is very probable that the perivisceral chamber 

 really forms part of the blood-vascular system." 



The Mollusca. 



In the Mollusca the circulatory system is more highly 

 differentiated, and " in very many, if not all, the blood cavities 



