90 



COMPARATIVE ANATOMY 



CHAP. 



for the inhalation and exhalation of air.i The ctenidium is placed to the extreme 

 right of the mantle cavity, a position which is in some way connected with the gi-eat 

 development of the pulmonary sac. It nevertheless answers to the left gill in other 

 Monotocardia, as can be seen from its innervation. 



3. The genus Valvata is unlike all other Monotocardia, in that its gill is 

 feathered on both sides and projects freely. It can, further, be protruded from 

 the pallial cavity. 



4. In Atlanta, among the Hcteropoda, the gill is well hidden in the spacious 

 mantle cavity. In Carinaria, it is only slightly protected in consequence of the 

 small development of the mantle fold. In Pterotrachea there is no mantle fold, and 

 the filamentous branchial leaflets project free and uncovered. Firoloides has no 

 gills. 



Opisthobranohia. — A true ctenidium is here found only in the Tedibranchia and 



Fio. 86.— General Morphology of the gills of the Prosohranchia. Diagrammatic sections 

 in the region of the mantle cavity, from behind. F A, Haliotis ; B, Trochus, anterior portion of the 

 pallial cavity. C, Trochus, inicklle or posterior portion of the cavity. D, Monotocardia. 1, 

 Mantle cavity ; 2, rectum or anus, r right, I left gill of Haliotis (A), which latter is the only gill 

 present in the AzygoiraiicMa (B, C) and i!oiwtocanUa (D). i, Branchial leaflet of the inner row ; 

 e, ditto of the outer row, between them the branchial axis or septum with the afferent and efferent 

 branchial vessels (8 and 4) ; 5, position of the mantle slit in Haliotis (cf. p. 43). Further explana- 

 tions in the text. 



in the Steganobranchia among the Ascoglossa. It lies, often incompletely covered, 

 in the mantle cavity which is developed on the right, and is, in some cases at least 

 (e.g. Pleurobranclius), distinctly feathered on both sides. 



In the Pteropoda, which must be derived from the tectibranchiate Opistho- 

 branchia, the ctenidium, when present, is little developed, and lies on the right side 

 of the body. It answers to the tectibranchiate ctenidium. 



In the Gymnosomata, this true gill is retained only in the Pneumodermidm as a 

 simple, or less frequently (Pneumoderma) fringed, process on the right side of the 

 body (Fig. 76, p. 79). New gills, on the other hand, may develop at the posterior 

 end of the body, occurring either together with the true ctenidium {Spongiobranchaa, 

 Pneumoderma), or alone (CHonopsis, Notohranclima), until they in their turn dis- 

 appear (Olione, Halopsyehe). 



Among the Thecosomata, the Cavolimidce alone (Fig. 87) possess a gill which 



^ In this and in the closely-allied Lanistes there is in addition 

 on the left side (v. Fischer and Bouvier, C R. cxi. p. 200). 



protrusible siphon 



