284 THE ANATOMY OF INVERTEBRATED ANIMALS. 



into lacunae between the organs, and into the general peri- 

 visceral cavity ; and, as in most Mollusca, ceasing to be con- 

 tained in vessels with distinct walls. 



The blood thus poured out eventually makes its way into 

 irregular sinuses or reservoirs, the chief of which, lodged in 

 the sternal canal, communicates by lateral channels with oth- 

 ers which lie above the bases of the thoracic appendages, 

 and from which the afferent branchial canals pass into the 

 stems of the branchiae, on the exterior faces of which they 

 ascend, giving off branches to the lateral filaments. Corre- 

 sponding canals return the blood from these filaments to the 

 efferent branchial canals, which run down the inner side of 

 the branchial stems, and unite above the bases of the limbs 

 into six trunks, which ascend beneath the epimera and open 

 into the sides of the pericardial sinus. The floor of this 

 sinus is formed by a continuous membrane, which appears to 

 shut it off completely from the general visceral cavity (at 

 least it retains air or fluid thrown into it), and, if this be 

 really the case, it may be said to be functionally a branchial 

 auricle, containing pure unmixed aerated blood. 



The branchiae are eighteen in number upon each side, and 

 are attached from the eighth to the fourteenth somites in- 

 clusively. Six of these branchiae are attached to the epipo- 

 dites of the eighth to the thirteenth somites, and differ very 

 considerably in appearance from the other twelve. Each 

 epipodite is, in fact, expanded at its upper extremity into a. 

 broad, bilobed membrane, which is folded upon itself, so that 

 the two lobes are directed posteriorly, and receive the epipo- 

 dite of the next limb (Fig. 71, N). The membrane of the 

 lobes is obliquely plaited, so that, doubtless, they subserve 

 respiration to a certain extent ; but, in addition, the anterior 

 edge of the epipodite is beset with a number of branchial 

 filaments, similar to those on the other branchiae. 



The latter (Fig. 71, JM,f, g) are simple plumes, consisting 

 of a stem, to which are attached many delicate, cylindrical 

 filaments. Two of these plumes are attached to the epimera 

 and coxo-epimeral articular membranes of the ninth, tenth, 

 eleventh, twelfth, and thirteenth somites. They increase in 

 size posteriorly. The eighth and fourteenth somites, on the 

 contrary, only carry one plume. A tuft of long byssus-like 

 filaments is attached to the coxopodite of each of the last 

 six thoracic appendages (Fig. 71, F, M). 



The respiratory organs of the Crayfish, not being pro- 

 vided with cilia, require some special arrangement for the 



