580 F. A. Potts, 



The Maiitle Diict. 



Uiider this title I refer to the tubulär structure running- ou the left 

 side of the body between the visceral mass and the mantle. It opeus 

 into the mantle cavity anteriorly and to the exterior at the apexofthe 

 bay. In sections (Figs. B, C) it is seen that the walls of the duct 

 are partly composed of a highly vacuolated tissue witli very small 

 nuclei, which appears to have a skeletal value {vac. m. d). But this 

 tissiie is not developed regularly but very thickly on the under 

 and anterior surfaces of the wall. As indicated, however, in Fig. A 

 the posterior part of the wall is much thinner and is made up of 

 fibrous cells. 



The lunien of the duct is considerably restricted. That it can 

 serve for the passage of larvae to the exterior is shown by the 

 presence of one or two in its course, one being shown in Fig. C 

 very near to the external aperture. But it seems impossible that 

 a ready egress can be ottered by its means to the thousands of 

 Cypris larvae which the brood pouch and its diverticula contain, and 

 especially when it is remembered that the feeble musculatnre of the 

 mantle cannot be very effective in forcing the larvae to the exterior. 

 In SaccuUna and Peltogaster. where a wide mantle aperture occurs, 

 the nauplii are expelled rapidly by the strong rhythmical con- 

 tractions of the well developed muscles of the mantle. But I feel 

 inclined to suggest that in this case there is a dehiscence of the 

 thin wall of the diverticula and that the larvae are thus liberated 

 with promptitude at the proper time. The mantle duct is without 

 doubt an essential organ in permitting the access of pure water for 

 the respiration of the larvae, and the musculature of the mantle 

 will sutfice for the slow pumping movements which subserve this 

 function. 



The internal aperture (Fig. B) is of a curiously irregulär 

 character. At the point represented in this section the external 

 wall has become merged in the mantle and the lumen of the duct 

 is observed as an irregulär slit opening towards the under surface 

 into the mantle cavity. The internal wall, with its vacuolated cells, 

 produced into a free forwardly prqjecting process which only ends 

 some distance in front of the aperture. 



The external aperture (Fig. C) is a small circular orifice 

 opening into the angle of the bay. 



