GASTROPODA. 



373 



M 



into irregular spaces in the tissues and amongst the organs, some 

 of which spaces have coalesced or dilated to give rise to the 

 perivisceral cavity (exclusive of the pericardium) of these animals. 

 From these systemic spaces the blood passes through -tjje branchial 

 (pulmonary) vessels to the respiratory organs. In some cases there 

 is, in addition to the ctenidial blood, a certain amount of blood 

 returning to the auricle from other parts of the mantle, e.g., 

 Acmaeidae, Heteropoda, Pleurobranchidae, Pneumodermatidae. Of 

 these the Pleurobranchidae, Hetero- 

 poda, and some Acmaeidae do 

 not possess secondary branchial 

 structures on the mantle; but in 

 other Acmaeidae and in the Pneu- 

 modermatidae there are secondary 

 branchiae in addition to the cteni- 

 dium. 



The blood is generally colourless ; it is 

 red in Planorbis (haemoglobin in the 

 plasma), and in certain forms has a blue 

 tinge (haemocyanin). There is a blood- 

 gland on the aorta in some Opistho- 

 branchs (Bn\]oids,Pleurobranchus, Dorids) ; 

 and in certain streptoneurous Platypoda 

 there is a corresponding organ near the 

 kidney, communicating with the auricle. 



Respiratory organs. In only a 

 small number of Gastropods is respi- 

 ration effected exclusively through 

 the general integument (some Nudi- 

 branchs). By far the greater number 

 breathe through gills, and many 

 through lungs ; a few combine 

 branchial and pulmonary respiration. 



The gills are either typical cte- 

 nidia contained in the mantle-cavity, or freely-exposed branched 

 processes of the dorsal integument (some Nudibranchs). 



Excluding the Chitonidae (Fig. 305), which are unlike other 

 Gastropods in having a number of pairs of ctenidia bearing two 

 rows of lamellae (i.e., bipennate) and placed in the hinder part 

 of the mantle-groove, there are never more than two ctenidia (Zygo- 

 branchs, Fig. 296) ; but usually an asymmetrical development takes 

 place, and only one gill (the left) remains (Fig. 280). 



TIG. 296. Dorsal view of Haliotis tuber- 

 culata after removal from its shell. 

 The roof of the mantle-cavity has been 

 cut through so as to expose the two 

 ctenidia, and the rectum D. The peri- 

 cardium also has been opened. K the 

 left ctenidium ; Dr the hypobranchial 

 gland; S the spindle - muscle ; F the 

 margin of the foot ; T tentacle ; eye ; 

 V ventricle ; A, A the two auricles 

 (from Glaus). 



