Respiration in Invertebrate Animals. 327 



collapses in a most remarkable manner. It is during the ex- 

 piratory act that the faecal excreta are expelled, and this takes 

 place through one and the same orifice. In this chai'acter the 

 Pulmonata are strikingly distinguished from the Pectinibran- 

 chiata. In the latter, the pure current entering the breathing- 

 chamber is scrupulously separated from that which is about to 

 be expelled. When water and not air is the medium of respi- 

 ration, this is a constant feature in the history of the Cepha- 

 lophora. 



This circumstance is still more beautifully and perfectly ob- 

 served in the physics of breathing as exemplified in the Cepha- 

 lopod mollusks (fig. 7). It has already been explained, that so 

 completely and intimately is the body of every Invertebrate 

 animal surrounded and apparently penetrated by the external 

 element, that not only is it profusely admitted into the digestive 

 and resjjiratory organs, but, as for example in tubicolous Worms 

 and testaceous Cephalophoi*a, its contact with the entire exterior 

 of the body is secured by express provisions. 



In the Cyclobranchiata, as formerly described, a water-reser- 

 voir occupies the concave apex of the shell : the abdominal coils 

 of the shells of all Univalves constitute a similar receptacle. 

 The same rule is recognized in the organization of the Cepha- 

 lopods. In this class, in a given time, a considerable volume of 

 water traverses the branchial chamber. The respiratory actions 

 of the mantle and the funnel are raj)id and powerful. 



In the Nautilus, Ammonite, and other testaceous Cephalo- 

 pods, the base of the branchial recess of the mantle is continuous 

 with the siphuncle. In this maimer the external element is ad- 

 mitted directly into the abdominal segments of the shell, therein 

 chiefly to subserve mechanical purposes. If the respiratory 

 chamber in the shelled Gasteropods were perforated at its pos- 

 terior border, opening thus into the spiral spaces of the shell, 

 the water occupying this portion in these families would stand 

 in the same relation to that of the branchial cavity as it does in 

 the case of the Nautilus. 



In those orders, chiefly the Dibranchiata, which are devoid 

 of an external shell, the respiratory chamber is larger and more 

 prolonged into the spaces between the vital organs than in those 

 in which this appendage is present. Octopus, Loligo, or Sepia, 

 afford the best opportunity for witnessing the mechanical actions 

 of breathing. The anterior edge of the mantle is separated from 

 the side of the body by a broad open fissure (fig. 7 b, and in- 

 going arrows). This fissure within the mantle assumes the 

 character of a canal which leads back along the floor {c) of the 

 branchial chamber as far as the attached or cardiac base of the 

 gills {d). Along this canal up to this point the water enters as 



