98 



COMPARATIVE ANATOMY 



CHAP. 



2. The Longitudinal Rows of Branchial Leaflets (Fig. 20, p. 13). These organs, 

 which lie to the right and left of the body in the Phyllidiidce and Pleurophyl- 

 Udiidce, bear the same relation to the (lost) true ctenidium as do the respiratory struc- 

 tures of the Patellidce above described to 

 the same organ, which in them is some- 

 times present, sometimes wanting. The 

 longitudinal rows consist of numerous 

 small lamellae which project from the lower 

 side of the enveloping mantle fold into the 

 shallow pallial cavity. There is either one 

 long row of these lamellae running along 

 the whole length of the mantle fold and 

 only interrupted anteriorly (Phyllidia}, or 

 a row interrupted posteriorly as well 

 (Plcurophyllidla) ; or again, the rows of 

 lamellae are confined to the posterior end 

 of the mantle fold (Hypobranchicea). The 

 genus Dermatobranchus has no gills. 



3. Dorsal Appendages (Cerata) (Fig. 

 18, p. 12). These processes vary very 

 much in form, being sometimes simple, 

 and sometimes branched ; they differ also 

 i^ivatly in number and arrangement. At 

 their tips there are often cnidophore sacs ; 

 these are invaginations of the ectoderm in 

 which stinging cells with stinging capsules 

 are developed. Diverticula of the intestine 

 (digestive gland) enter the cerata, and may 

 open outward at their tips. The cerata are 

 generally striking and beautiful both in 

 F Fin. us. Respiratory and circulatory colour and markings. In some cases they 

 system of Doris, alter Leuckart (" Wand- may serve for protection and concealment, 

 tafeln "). a, Rhinophore ; b, posterior edge of in others, where the brilliant colouring is 

 the visceral dome ; c, end of the foot ; d, plumose comb i ne d with stinging properties, they 

 gills ; rfj, two gills cut oft'; c, anus ; /, auricle ; tt * 



?/, vntricle ; h, aorta ; i, circular vein around ma y se *' ve as a warning. They often break 

 the anus, which receives the arterial blood from off easily at the base (as a protective ar- 

 the gill, and sends it through the branchial vein rangement), and are always quickly regen- 



into the auricle ; k, circular artery, which receives 

 the venous blood coming from the body ; x, two 



crated. They no doubt assist, like the 



V**V VWW0 ftWU WAUCAUK 1IVJI11 L11C UUV1V . ,/-, VCTV j_ f j_l 1 J / 



vascular trunks, which conduct venous blood rest f the bod y SUrface > m Aspiration, 

 direct to the heart. especially where they are much branched 



and richly supplied with blood-vessels. 



Certain Opisthobranchia are altogether gill-less, e.g. the Elysiidce, Limapontidce, 

 and Phyllirrhoidce. 



Among the Pulmonata, the shell- less genus Onchidium has developed adaptive 

 gills. The species of this genus are amphibious, living on the sea-coast, within reach of 

 the tide. Their pulmonary cavity is very small ; respiration therefore takes place by 

 means of the richly vascularised dorsal integument, and especially of the simple or 

 branched dorsal papillae, in which there is a rich vascular network, which receives 

 the blood from an afferent vessel and gives it off to an efferent vessel. 



