Respiration in Invertebrate Animals. 325 



tions in this one particular ! The structure of the organs con- 

 tained in these variously located chambers will be found to differ 

 no less remarkably. 



In the Pleurobranchiata the fuiTOW for the branchia is situated 

 between the foot and the lateral free border of the mantle. It 

 differs in no essential respect in character and locality from that 

 of the Patellidse. This crypt or fossa has a higher position on 

 the side in the Aplysiadse. The mechanics of this cavity follow 

 the same principle in all. The water-currents bear in an inward 

 and backward direction under muscular and ciliary agency. 



In the Pectinibranchiata (fig. 3 A, 3 B) a distinct and well- 

 defined thoracic chamber exists («, a). It is situated on the 

 antero-dorsal region of the animal, and fills the anterior coil of 

 the shell. It is overvauked by the mantle. It does not form a 

 closed cavity as in the Pulmonifera (figs. 4, 5, 6). It opens in 

 front by a fissure extending from the right angle to the left. 

 Behind, it is closed by the adhesion of the mantle to the edge of 

 the diaphragm-like (fig. 4 b) partition between this chamber and 

 the abdomen, thence the mantle is prolonged in a thinner form 

 over the latter region. In this water- breathing order of mol- 

 lusks this chamber is not exclusively specialized to the office of 

 respiration. In every family it contains the termination of the 

 intestine. It is thns at once respiratory and cloacal. In its 

 walls, at a point differing in different families, is situated the 

 heart, and a system of glands of complex formation. The 

 branchiae form only a small integral constituent. In the physics 

 of this cavity one plan prevails throughout the Pectinibranchiate 

 order. The branchise occupy a point in the chamber opposite 

 to that taken up by the rectum. The former are to the left, the 

 latter lies on the extreme right. It is on this side also that all 

 the other excretory ducts terminate. The water- currents are 

 excited and sustained by the muscular action of the parietes of 

 the chamber. This force, which repeats itself in a regulated 

 order, constrains the water to move in a fixed and determinate 

 direction. It enters first at the left side, through the siphon 

 (fig. 3 B, arrows), if this appendage be present — through the 

 fissure directly, if it be absent. This pure stream impinges im- 

 mediately upon the branchiae. At this point, by means of an 

 exquisite concert of muscular and ciliary forces, the mass of 

 water thus received is divided into as many vertical sheets or 

 secondary colunms as there are spaces between the branchial 

 leaflets. As the cartilaginous edges of these laminae are pro- 

 vided with muscles, and the flat surfaces of each are strewn with 

 cilia (as will be afterwards described), the water is subdivided 

 again into myriads of invisible streamlets. Issuing from the 

 intcrlauiinar spaces where its course is slow, the dispersed 



