VII 



MOLL USCA—PARAHITIG GASTROPODA 



245 



with tlie outer world only by means of a small aperture at the tip of the swelling. 

 The parasite, thus established in the integument of its host, is surrounded on all 

 sides by a fleshy envelope {sm). This envelope is only broken through by an 

 aperture at the point where the apex of the dextrally twisted shell lies ; this 

 aperture corresponds in position with the aperture above mentioned as occurring 

 at the tip of the pathological swelling. Tliis envelope is called the false mantle, 

 and corresponds morphologically with the false foot of Thyca, much increased in 

 size and bent back on to the shell. There occur besides a true mantle, a gill, a 

 rudimentary foot without an operculum, eyes, auditory organs, and a typical 

 Prosobranchiate nervous system. The development of the remarkable false mantle 



Fig. 203,— Longitudinal section tlirougli Stlllfer Llnoklse (after P. and F. Sarasln). 6c, 

 Buccal ganglion ; H, blood sinus ; mr, cerebral ganglion ; d, alimentary canal ; fs, foot ; fc, gill ; I 

 liver ; ml, mantle ; n, proboscidal nerve ; oo^ eye ; ot, otocyst ; ^ed, pedal ganglion ; pr, proboscis ; 

 s^n, false mantle ; sut, subintestinal ganglion ; sit^, supraintestinal ganglion. 



no doubt signifies that, although the animal is embedded deep in the integument 

 of the host, communication with the exterior is retained. Water for respiration can 

 enter and flow out of the mantle cavity, and the faecal masses and genital products, 

 and perhaps also the larvse can pass into the cavity of the false mantle and be 

 ejected through its aperture. The sexes are separate. The snout has lengthened 

 into a very long proboscidal tube which pierces the tissues of the integument of the 

 Star-fish, which are rich in blood, and draws from them the necessary nourishment. 

 -Both oesophageal bulb and radula are wanting. 



3. The two parasites just described are typical Gastropods, and are easily 

 recognised as such when carefully examined ; there are, however, two other 

 parasitic Gastropods in which the typical organisation is so much modified that it 



