294 Introduction to Animal Morphology. 



droidal processes, of which there are never more than 

 two sets, and often one (the left) is rudimental. 



Rarely are they symmetrical, as in Patella, Chiton, &c., 

 between the foot and the edge of the mantle (Cyclobranchi- 

 ata). In Haliotis the two gills are displaced to the one side. 

 They may be in a special mantle cavity (Aplysia, Bulla, Pro- 

 sobranchiata), into which a siphonal gutter, never a complete 

 tube, may lead. The gills are sometimes uncovered when 

 the mantle is absent (Gymnobranchiata), and then they may 

 be as a row of dorsal papillae (Cerabranchiata*), each of which 

 has a separate circular muscular coat, and often a nest of 

 cnidae at the tip. In Scyllaea, Tethys, Dendronotus, &c., the 

 dorsal processes are branched, tree-like, in two rows, and with 

 no liver-caeca (Polybranchiata). Sometimes they form a dor- 

 sal circlet or wreath round the anus (Pygobranchiata, as in 

 Doris, sometimes only half surrounding it, crescentic, as. 

 Hi -ptabranchus). In most Heteropods the gills form a clus- 

 ter of thread-like or leaf-like organs near the point of the nu- 

 cleus and beside the heart, between the anus and ciliary 

 organ (Pten .trachea) ; sometimes they lie in a mantle cavity 

 within a shell (Atlantidoe). In this group the gills are simple 

 enlargements of the body wall covered with a thin dermal 

 and muscular layer, with no proper branchial vessels (Nucleo- 

 branchiata), whereas in the others the gills are special organs. 

 In Elysia, &c., the gills are flat lateral expansions, sometimes 

 radially folded (Placobranchiata). In some of these Gymno- 

 branchiate forms the axis of the gill stem has a firm elastic 

 skeleton, composed of a double row of flattened hollow glo- 

 bules piled on top of each other. In the shell-bearing Opis- 

 thobranchs and in Prosobranchs the gills lie under the mantle 

 lobes, and may be symmetrical on both sides (Dipleuro- 

 branchiata), or obsolete on one side (Pomatobranchiata). In 

 the last form the shapes of the gills are very variable, and 

 there may be one (Umbrella), two, or three branchial veins 

 to the auricle, with or without special walls. In Prosobranchs 



* The lateral appendages of Dorideans (Ancula, Triopa) are not 

 branchial, but only dermal processes. 



