No. 3-] THE MUSCULATURE OF CHITON. 6ll 



external openings of the reproductive and excretory organs ; it 

 is protective and is used also as a water course in respira- 

 tion and during the discharge of eggs and spermatozoa, as 

 described by Metcalf.^ " During the ordinary respiration of 

 the Chitons, at one anterior point, and at a posterior point on 

 the opposite side, a small tube is formed by the arching up of 

 the mantle edge, the bottom of the tube being formed by 

 whatever surface the mollusc is resting upon. A constant 

 stream of water passes into the anterior tube, through the 

 mantle chamber and out of the posterior tube. During the 

 discharge of the sexual products, instead of one there are two 

 posterior tubes, one on each side, in the region of the orifice of 

 the oviduct or of the vas deferens as the case may be. The 

 eggs, or spermatozoa, are carried out of the mantle chamber 

 through these tubes by the ordinary respiratory current. At 

 other times during ovulation, the whole posterior part of the 

 mantle of the female would be raised from the floor of the 

 aquarium and the eggs allowed to pass out freely through the 

 wide space thus formed." 



The most prominent muscle, the " interior mantle muscle " 

 (Figs. 4, 5, 7, etc., tin), passes from the ventral surface of the 

 shells where the mantle joins the body, and occupies the part 

 of the mantle that borders immediately upon the mantle cham- 

 ber ; the muscle is found in every part of the mantle, around 

 the entire body. To the median shells it is attached almost 

 continuously along a line over the highest part of the mantle 

 chamber (Fig. 3, hna, Fig. 4, hn), and immediately anterior to 

 the apophysis of each shell, the fibres reach farther in under 

 the shell than in any other regions, so that they meet and 

 even cross the most lateral of the medio-pedal fibres of the 

 posterior group {cf. Fig. 3). As the apophysis broadens poste- 

 riorly, the fibres become continuous with those at the antero- 

 lateral edge of the cushion of muscles dorsal to the apophysis 

 (Fig. 10, im and ci). Still more posteriorly the fibres assume 

 their attachment to the ventral surface of the apophysis, the 

 dorsal shell being no longer accessible because of the greater 



^ Contributions to the Embryology of Chiton. Studies from the Biological 

 Laboratory of Johns Hopkins University, vol. V, No. 4. 



