98 THE GASTROPODA 



by haemoglobin in a fairly large number of Streptoneura, e.g. 

 Littorim, Bucdnum, Natica, etc. In some few Gastropods the blood 

 is of a bluish tint in consequence of the presence of an albuminoid 

 containing copper called haemocyanin. In other cases the blood is 

 coloured by pigments of extraneous origin absorbed by the amoe- 

 bocytes ; this is the origin of the violet-red colour of the blood of 

 Fasciolaria. In various Opisthobranchs, viz. Bullomorpha, Pleuro- 

 branchidae, Doridomorpha (Fig. 79, XVII), there is a differentiated 

 lymphatic gland, situated as a rule more or less anterior to the 

 heart on the aorta. In a certain number of Streptoneura this 

 organ consists of a sinus filled with cytogenous connective tissue 

 and situated near the kidney ; in other cases it is diffused through 

 the subcutaneous connective tissue. 



The heart is always dorsal and in the immediate neighbourhood 

 of the respiratory apparatus (Figs. 79 and 82). It is only in the 

 very archaic forms such as Pleurotomaria and the Fissurellidae, that 

 it is still symmetrical and median as in the Cephalopods, Lamel- 

 libranchs, and Amphineura, otherwise it is nearly always lateral, 

 being situated on the left in dextral forms (Fig. 67). It is generally 

 somewhat anterior in position (Figs. 82 and 88), but it may become 

 posterior again as a result of secondary specialisation as in Pterotrachea 

 (Fig. 143), Testacella, Oncidium, Peronia, and the Doridomorpha, and 

 in the last named it resumes an apparent external symmetry 

 (Fig. 79). The heart of Gastropods always includes an ovoid or 

 piriform ventricle, and in the Rhipidoglossa (with the exception 

 of the Helicinidae, Hydrocenidae, and Proserpinidae) two auricles, 

 but the latter only retain their primitive symmetry in the Fis- 

 surellidae, in which the ctenidia themselves retain their symmetry 

 and the heart is median. In other Rhipidoglossa, in which the 

 heart is no longer median, the right auricle is the smaller (Fig. 

 55, VII), and it becomes more and more rudimentary. In all other 

 Gastropods there is only one auricle, situated on the topographically 

 left side (Fig. 82, au) : it is generally larger than the ventricle, but 

 its muscular fibres are fewer in number and its walls are thin, 

 transparent, and extensible. The ventricle is traversed by the 

 rectum in the Rhipidoglossa (except the Helicinidae), and in the 

 more archaic forms is placed between the two auricles, e.g. in Pleuro- 

 tomaria (Fig. 127, h), Trochus (Fig. 55, VIII), etc. In the majority 

 of the Streptoneura (Fig. 99, V), in the Pulmonates (Fig. 86, 

 VII), and in some Bulloinorpha e.g. Actaeon, Limacina (Fig. 63), 

 Clio virgula, and Clio acicula this ventricle is posterior to the 

 single auricle; in some Opisthobranchs (Phyllirhoe, Fig. 161) and 

 Heteropods the auricle and ventricle are on the same transverse 

 line, and in the majority of Opisthobranchs (Figs. 79, III, and 

 92, I), the Testacellidae, Oncidiidae, Pterotracheidae, and certain 

 Calyptraeidae the ventricle is in front of the auricle. In adult 



