392 INVERTEBRATA CHAP. 



that is to say, he thinks that it affords a convenient insertion for 

 muscle fibres ; those inserted in its upper part he regards as retractors, 

 the encircling fibres as protractors, and the vibratile plume he con- 

 siders the real sense-organ. He believes that this sense-organ comes 

 into play just before the fixed life is taken up, and that its function 

 is to select a suitable spot for the fixation of the larva. 



The internal sac, or sucker, arises just in front of the anus, and, 

 according to Kupelwieser, it begins as a solid thickening of the 

 ectoderm, which soon splits into two layers separated by a cavity. 

 Prouho, however, says that it arises as an invagination just in front 

 of the anus. This little sac develops, as larval life proceeds, into a 

 wide, spacious sac, which is drawn out into two horns. The upper 

 wall of the sac remains thin, but its lower wall, where it abuts on 

 the atrial cavity, becomes glandular, and produces great masses of a 

 slimy secretion. Eventually this wall breaks down and allows the 

 sucker to open into the atrial cavity, into which the secretion is then 

 discharged. Two muscles arise, one on each side, from the ectoderm 

 of the central parts of the flat sides of the larva, and are inserted 

 into the upper,wall of the sac. These muscles, termed the sucker 

 muscles, only come into play at the metamorphosis. 



The ectoderm of the sides of the larva, as we have already noted, 

 secretes two thin valves of shelly material, which thus form a 

 bivalve shell protecting the larva. Round the edges of each triangular 

 shell-bearing area there runs a cushion of large swollen cells, filled 

 with a mucoid secretion, thus taking on the outline of a triangle. 

 From the base of this triangle a ridge of the same material projects 

 upwards a short distance. The ectoderm covering the narrow sides 

 of the larva, between the valves, is covered with short cubical ciliated 

 cells. 



From this description it follows that the apical organ is bounded 

 laterally by the cell-cushions, and front and back by cubical epithelium. 

 The apical organ itself is a two-layered, slightly concave plate, or 

 shallow cup of cells. The rim of this plate is composed of converging 

 columnar cells, each bearing a single stiff cilium or sense -hair. 

 Inside this outer ring comes a second ring of cells bearing pigment, 

 to which Kupelwieser assigns a visual function, while in the centre 

 there is a mass of clear rounded cells. From the fact that a bundle 

 of nerve fibres proceeds from these inner cells, Kupelwieser draws 

 the conclusion that they are of a ganglionic nature. 



The fibres of the dorsal muscle penetrate between the cells of the 

 apical organ in order to attain their insertion ; but not all the fibres 

 of the dorsal muscle, in fact only the minority, have this insertion. 

 The majority of the fibres of the dorsal muscle pass back under the 

 apical organ and diverge into bundles, right and left, and are inserted 

 into the ciliated cells of the posterior part of the corona. Most of 

 the fibres belonging to the dorsal muscle are striated, but some 

 muscles have smooth fibres. 



There is an adductor muscle connecting the two valves of the shell 



