244 



COMPARATIVE ANATOMY 



CHAP. 



Rossia, it is the first arm which is hectocotylised. Finally, both the arms of one 

 pair may be thus transformed ; in Idiosepiou and Spirula this is the case with the 

 fourth pair, in Eossia with the first. 



The difference in size between the male and the female, which has been mentioned 

 as occurring in those forms which have true hectocotylised arms, is also found, 

 though not to the same degree, in many other Cephalopoda, in which the male is 

 slightly smaller than the female. 



XXI. Parasitic Gastropoda. 



1. Thyca ectoconcha (Fig. 202) is a Prosobranchiate Gastropod which is parasitic 

 on the Star-fish Linckia miiltiforis. The chief points in its organisation are shown 

 in Fig. 202, a longitudinal section in which, however, several organs which lie 

 laterally to the section are also represented. The organisation of the Gastropod is as 

 yet little influenced by its parasitic mnnner of life. It possesses a shell, shaped 



¥ic,. 202.— Longitudinal section through Thyca ectoconcha (after P. and F. Sarasin). Some 

 organs not actually belonging to tlie section are included, cer, Cerebral ganglion ; rf, alimentary 

 canal ; fl, folds ; fs, foot ; /,-, gill ; I, liver ; iiil, mantle ; oc, eye ; ot, otocyst ; pcd, pedal ganglion ; pr, 

 proboscis ; .<■/, false foot ; si, resopliageal bulb ; vl, cephalic fold. 



somewhat like a Phrygian cap. In the mantle cavity lies the gill. The alimentary 

 and nervous systems also are in no way remarkable. It has eyes and auditory 

 organs, and a short powerful snout, and muscular oesophageal bulb, which penetrates 

 the Star-fish between the calcareous parts of its integument into the tissues. There 

 is no radula. The base of the snout is surrounded by a muscular disc consisting 

 of an anterior and a posterior part. This disc, the so-called false foot, is the 

 grasping organ by which the animal attaches itself to the integument of its host 

 so firmly that it cannot be torn away without injury. The rudiment of a foot 

 [fs) occurs without an operculum. 



2. The Gastropodan organisation is somewhat more strongly modified in 

 Stilifer Linckice (Fig. 203), which is parasitic on the male Linckia. The whole 

 body of this parasite penetrates into the calcareous layer of the integument of the 

 host, on which it raises pathological globular swellings, and further causes the 

 peritoneum to bulge inwards towards the body cavity. The parasite communicates 



