144 Dr. T. Williams on the Mechanism of Aquatic 



in the land species, and to guard against the injurious entry of 

 water in the aquatic tribes*. Hence they have been called 

 Adelo-pneumona (or concealed-lunged) by Dr. Gray ; the Opercu- 

 lata, by antithesis, being named the Phanero-pneumona or open- 

 lunged. 



The Onchidiadse are sea slugs, breathing by means of a pul- 

 monary cavity, but living immediately in contact with marine 

 conditions. According to the dissections of Mr. Hancock, the 

 " lung is placed in this group at the posterior extremity of the 

 body, and has consequently the heart in front of it/^ 



Respiration in the Limacidse is accomplished by means of a 

 cavity seated on the back near the neck, and covered by the disc 

 (PI. XI. fig. 1). It opens on the right by a valvular sphincteric 

 orifice, which is endowed with an active power of widely dilating 

 and of closely contracting [e). In this family the rectum does not 

 traverse the respiratory chamber ; it lies external to, and below 

 its right boundary. The anal orifice therefore is separate from 

 that of the respiratory ; it is seen immediately below the latter. 

 This is a fact of structural disparity between the Limacidse and 

 the Helicidse. In the latter the intestine is a prominent object 

 in the breathing-chamber, and the vent is confounded with the 

 pulmonary orifice. The position of the generative outlet is 

 variable. 



The following description is founded upon numerous dissec- 

 tions of the common Slugs. The breathing-sac presents in all 

 the species the same anatomical characters. It is best studied 

 by fixing the animal with two strong pins, transfixing the body 

 in front near the head and behind near the tail in a cork-bottom 

 dissecting dish. One blade of a strong, blunt-pointed sharp 

 scissors should be inserted into the pulmonary orifice. The 

 pomt should now be carried round the boundaries of the chamber, 

 the disc being cut as the instrument travels round. Such a 

 section (fig. ] ) will enable the lid of the cavity or the respiratory 

 disc {b) to be so perfectly raised as to leave the entire space 

 underneath quite uninjured. A part corresponding with the 

 pericardium (c) will however be found to be adherent to this 

 roof; it should be snipped with the scissors. The roof, in the 

 substance of which is lodged the rudimentary shell, should now 

 be reflected and pinned down (as shown in figs. 1 & 2). From 

 the central space of the cavity a second membrane (fig. 1 c) will 

 require to be raised. This is much more vascular than the 

 former integumentary covering {b), and constitutes really a part 

 of the respiratory surface. This structure serves also as peri- 



* See the excellent Manual on Recent and Fossil Shells, by S. P. Wood- 

 ward. 



