98 



COMPARATIVE ANATOMY 



CHAP. 



Of 



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 Phyll idiidcc and Plcurophyl- 

 lidiidce. 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 lamellfe running along 

 the whole length of the mantle fold and 

 only interrupted anteriorly (Phyllidia), or 

 a row interrupted posteriorly as well 

 (Pleurophyllidia) ; or again, the rows of 

 lamella? are confined to the posterior end 

 of the mantle fold (Hypobranchicea). The 

 genus Dcrmatobranchus 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 

 greatly 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 



FK:. 93. - Respiratory and circulatory eolour alld markings. In some cases they 



system of Doris, after Leuckart ("\Vuml- may serve for protection and concealment, 



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



the visceral dome ; c, end of the foot ;d, plumose com bined with stinging properties, they 

 gills ; tli, two gills cut oft"; e, anus ; /, auricle ; 

 rj, ventricle; 7i, aorta; i, circular vein around 



the gill, and sends it through the branchial vein 

 into the auriide ; k, circular artery, which receives 

 the venous blood coining from the body ; x, two 

 vascular trunks, which conduct venous blood 

 direct to the heart. 



may serve as a warning. They often break 



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

 rangement), and are always quickly regen- 

 erated. They no doubt assist, like the 

 rest of the body surface, in respiration, 

 especially where they are much branched 

 and richly supplied with blood-vessels. 



Certain Opistkobranckia are altogether gill-less, e.g. the Elysiida:, Limapontidce, 

 and Phyllirrhoida;. 



Among the Pulmotmta, 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. 



