130 ELEMENTARY BIOLOGY. [xir. 



long pseudopodial prolongations, and thereby take an irregu- 

 larly stellate form. 



It has been seen that the respiratory organs, or branchia3, 

 are lodged in a chamber situated between the branchiostegite 

 externally, the lateral walls of the thoracic somites internally, 

 and the bases of the thoracic limbs below; and that there is 

 a narrow interspace between the free edge of the bran- 

 chiostegite and the latter. At the anterior end of the cham- 

 ber, a funnel-shaped passage leads to the anterior opening 

 mentioned above, and, in this passage, the scaphognathite lies 

 like a swing door. 



During life, the scaphognathite is in incessant movement 

 forwards and backwards, scooping out the water in the bran- 

 chial chamber through its anterior aperture at every forward 

 motion. The place of the water thus thrown out is taken by 

 water which flows in by the inferior and posterior cleft be- 

 neath the free edge of the branchiostegite, and thus a constant 

 current over the gills is secured. Each branchia is somewhat 

 like a bottle-brush, having a stem beset with numerous fila- 

 ments ; and the blood contained in the vessels of the latter 

 being separated by only a very thin membrane from the air 

 contained in the water, loses carbonic anhydride and gains a 

 corresponding amount of oxygen in its course through the 

 branchiae. 



The branchiae are attached partly to the epimera of the 

 -thoracic somites, partly to the proximal ends of the thoracic 

 limbs. The epipodites of the limbs ascend between the sets 

 of branchiae which belong to each somite, and separate them. 

 The branchiaB which are attached to the limbs must neces- 

 sarily be stirred by the movement of the latter, and hence the 

 exchange of gases between the blood which they contain, and 

 the water must be, to a certain extent, increased, in proportion 

 to the muscular contractions which give rise to the movements 



