TOPOGEAPHICAL EELATIONS AMONG THE DOCOGLOSSA. 277 



Part III. — The Visceral Hump of the Docoglossa. Introductory. 



With the shortening of the foot the mass of the visceral hump would come to lie quite 

 behind it— that is, with its weight operating against the force of adhesion. In such a 

 condition of the spiral, too, the waves would have great power. Variations of o-rowth 

 would therefore be selected, in which the extension of the shell-edge was especially 

 marked in the posterior region. In this way the spiral or its remains would acquire 

 a position over the posterior part of the shell-mouth, and a continuance of the process 

 gives the condition in very young AcmcecB (PL 15. fig. 2). Later, even this vestige of 

 the spiral disappears, and the well-known Docoglossan cone-like shell remains; the 

 disappearance has undoubtedly increased efficiency of adhesion. The cone-shaped shell 

 is highly advantageous under the conditions of life, for a large part of the pressure of 

 the waves glances oflf without appreciable effect, and another part by strikino- down- 

 wards only strengthens the resistance to removal. 



As the shell-rim grew backwards, a space remained above the broadening foot which 

 became occupied by the posterior part of the contents of the spiral. The contents of the 

 apical region of the spiral, now fau-ly far forwards, settled down in the upper part 

 of the cone (figs. 3-5) — that is, above the contents of the base of the spiral. This may be 

 called the first stage of the process of consolidation of the viscera. Its occurrence is 

 inferred partly from the topographical relations of organs to be considered later, and 

 partly from a consideration of Boutan's figures of embryos of AcmcBa. 



It is of interest to note that enlargement of the shell-rim, so that the spiral no lon^-er 

 extends beyond it, is also found among the Bellerophontidte (fig. 1). 



Part IV. — Brancdial Cavity and Heart. 



Tlie slit in the anterior edge of the shell weakened the force of attachment, and 

 natural selection operated to bring about its disappearance, though this reduced the 

 efficiency of the branchial cavity. The settling of the viscera as just described also 

 reduced its efficiency by pressing on the cavity from behind. The apical viscera 

 accommodated themselves on the right side (fig. 7), as the left ctenidium was by far tlie 

 more important of the two, and free space for it was therefore a necessity. This 

 rendered the right ctenidium still more inefficient, and it disappeared, the anus and 

 excretory openings meanwhile shifting to the place it left vacant (PI. 17. figs. 19 & 20). 



The respiratory arrangements have been discussed by Dall, and are the bases of the 

 modern classification of the Docoglossa. These arc, therefore, relatively weU known, 

 and need not be described at length on this occasion. 



Pressure of the viscera reduced the branchial cavity, and led to selection of variations 

 whereby the position of the remaining ctenidium became oblique, and a greater length 

 could be sheltered in the cavity, while a larger surface was thus turned to the incoming 

 stream. Considerations of space meanwhile led to the reduction of the mucous glands, 

 and these two changes greatly increased the chances of damage to the ctenidium due to 

 excrement. These disadvantages, together with the pressure on the branchial cavity, its 

 imperfect currents, and the frequent prolonged exposure to air, brought about the 



SECOND series. — ZOOLOGY, VOL. IX. 39 



