PHYLUM MOLLUSC'OIDA 



351 



used for prehension. In the Phylactolauuata (Fig. 280) the 

 lophophore is horse-shoe-shaped, in the Gymnokvmata (Fig. 276) 

 circular : in the former, but not in the latter, there is a ciliated 

 lobe, the epistome (Fig. 282, ep) — which may have a sensory func- 

 tion — overhanging the iQOuth on the anal side. The retraction oi 

 the introvert is effected by a pair of bands of muscular fibres, the 

 parieto-vaginal muscles, passing to it from the body-wall, and 

 by a pair of retractor muscles passing from the latter to the ali- 

 mentary canal. 



Structure of body-wall. — Beneath the cuticle is an epi- 

 dermis, consisting of a single layer of flattened polygonal cells, 

 firmly united together by their edges. Beneath this there 

 is usually, but not always, a layer of muscle, which is 

 arranged in two strata — an 

 external composed of circular, 

 and an internal of longitu- 

 dinal fibres. There is an ex- 

 tensive coelome lined in some 

 forms (Phylactolsemata) by a 

 definite coelomic epithelium, 

 in part ciliated; while in 

 others there is no such de- 

 finite epithelium, but its place 

 is taken by thin parietal and 

 visceral layers of an irregular 

 cellular tissue — the paren- 

 chyma. Crossing the ccelome 

 are strands, in some instances 

 very numerous, of spindle- 

 shaped cells. In some cases 

 two mesenteric bands sus- 

 pend the alimentary canal — 

 an anterior attached near the 

 mouth and a posterior passing 

 from the caecum to the aboral 

 cases the latter, to which the special name of funiculus is given, is 

 alone present. 



The alimentary canal has in all species the parts that have 

 been already described in the case of Bugula. In some of the 

 Cheilostomata it is stated that the cells of the oesophagus bear 

 numerous striated muscle-fibre processes. In some Ctenosto- 

 mata there is in addition a thick-walled chamber — the gizzard 

 — with chitinous teeth, between the CESophagus and stomach. 



The nervous system consists of a single, sometimes bilobed, 

 ganglion (Fig. 280, gang, and Fig. 282, ga) placed between the 

 mouth and the anal aperture, and of nerves passing firom it 

 to the various parts. There are never any organs of special 



Fio. 282.— Anterior portion of the body of 

 IjOphopuS, from the right side. an. anus ; 

 ep. epistome ; ga. ganglion ; o. rnouth ; pr. in- 

 testine ; St. oesophagus ; t. tentacles, cut off 

 near tlie base. (From Lang's Comparative 

 Aiuito'iinj. After Allraan.) 



end of the zooecium ; in most 



