Respiration in Invertebrate Animals. 151 



carefully from its .shell (tigs. 4 & 0). The ti.xed position afforded 

 by the ligament which tics the body to the columella, yields im- 

 portant service in the mechanical acts of respiration. 



The soft abdominal segment (tig. 3 j) of the body is covered 

 by a continuation of the mantle. In this situation the mem- 

 brane is thinner, smoother and more delicate. // is quite adherent 

 everywhere to ilie subjacent organs. There are no vacuoles either 

 between it and the invested viscera, or between the viscera them- 

 selves. During retraction the foot may be concealed completely 

 in the cavity of the thoracic moiety of the animal. This ex- 

 plains why it is that the viscera (brain, oesophagus, portions of 

 reproductive and chylopoietic viscera, &c.) are so loosely packed 

 in this region, and why it is that large spaces filled with fluid 

 lie intermediately. Such vacuoles are more spacious in the lleli- 

 cidffi than in the Limaeida?, because in the former the head and 

 foot are more retractile than in the latter. The anterior surface 

 or front of the operculum is perforated on the right side by a 

 large, valvular, irritable sphincteric orifice (figs. 4 & 5 a, a). In 

 the edge of the mantle directly above this orifice is observed a 

 deep notch (fig. 4 e), which, when the animal is tightly coiled 

 up into itself, fits over the orifice. By this simple contrivance, 

 under all circumstances, the patency of the communication 

 between the breathing-chamber and the external air is secured. 

 So important is this point, that, both during hybernation and 

 when the animal remains long attached to a dry calcareous stone 

 in arid seasons, the membranous epiphragm which is then formed 

 from the mucus supplied by the mucus-gland, is valvularly 

 perforated at a point corresponding to the respiratory orifice. 

 Respiration therefore, though sometimes greatly reduced in 

 amount, at no time during the life of the animal completely 

 ceases. 



The pulmonary plexus, which in Helix is restricted to the roof 

 of the cavity (fig. 3 h), presents a much more regular and sym- 

 metrical arrangement of the vessels than that of Limaa\ In 

 Helix a main vessel [d, d) runs obliquely from left to right along 

 the vault of the cavity ; it terminates by dilating into the auricle 

 (c) ; it commences at the anterior border {d') in branches which 

 converge upon it with great regularity of course. The lateral 

 trunks are similarly regular. In some places the ultimate vessels 

 can be traced with the naked eye : they are best viewed as 

 opake objects, by cutting off the entire roof and placing it, 

 vessels uppermost, between two slips of glass, and then examining 

 with a two-inch or an inch object-glass. It will be observed that 

 the primary or large trunks (fig. 6 a, a, a) run, on the whole, in 

 parallel directions, enclosing interspaces of pretty uniform dia- 

 meters ; and that the secondary branches [h, h) proceed from the 



