Respiration in Invertebrate Animals. 145 



cardium. It overcovers the heart, and embraces the large curved 

 gland, in the centre of the area bounded by which the heart 

 (PI. XI. fig. 1 d) is situated. A perfect view of this pulsatile 

 body is thus obtained. It beats slowly, about 30 or 35 times 

 in a minute ; less frequently probably in an unmutilated animal. 

 It consists of an auricle which is situated under or below the 

 ventricle. The ventricle is not a linear continuation of the 

 auricle, separated only by a constriction as it is in the Helicidse, 

 but a separate organ receiving the auricle by a laterally placed 

 auriculo-ventricular orifice. It will be seen that the large blood- 

 channels (fig. I g) which are distributed over the floor of the 

 cavity, and which in nature are of an opake white colour, con- 

 verge upon the auricle, and that the aorta (A) rising boldly up 

 from the ventricle as a vigorous vessel of considerable calibre, 

 pierces the base of the chamber and disappears, travelling back- 

 wards among the viscera. 



Now of the Limacidse it cannot with truth be said that they 

 are either Proso-branchiate or Opistho-branchiate. The heart 

 here is certainly not placed either before or behind the respi- 

 ratory organ, but in its true centre. The ovoid space circum- 

 scribed by the gland (c/, d), and roofed down by the pericardium, 

 has only one opening through which the air can efiiect its ingress 

 and egress. This opening corresponds to the point at which 

 the line of the gland is interrupted. In some species it is placed 

 at a point on the left side remote from the external orifice (e), in 

 others behind ; in others again it has an anterior position. That 

 portion of the cavity which is without the gland, and between it 

 and the outer boundary of the disc (/,/), forms a circle, so that the 

 air may course around the cavity from right to left or reversely, 

 according to the tendency of the muscular action by which it is 

 impelled. The movements of breathing are far less manifest in the 

 slug than in the snail. In the slug the pulmonary orifice slowly 

 opens, and the bag is emptied by the slow expulsion of the air. 

 The act of inhalation is performed in a similarly slow manner. 

 The orifice now firmly closes, and remains closed until the next 

 act of expiration, which may occur irregularly in ten minutes or 

 a quarter of an hour. 



Two points in the history of the respiratory cavity of the 

 Limacidse demand special attention. First, the structure and 

 distribution of the vessels, and secondly, the character of the 

 membrane or covering by which the cavity is lined. As a rule, 

 it may be stated that the pulmonary vessels are distributed in 

 the Limacidse over the floor of the chamber, in the Helicidse 

 over the roof. In the former the mucous gland is considerably 

 larger than in the latter. The heart of Limax lies, like the pul- 

 monary network, on the bottom of the cavity ; that of Helix y 



Ann. §• Mag. N. Hist. Ser. 2. Vol. xvii. 10 



