VII MOLLUSCA— INTEGUMENT, MANTLE, VISCERAL DOME 41 



called) of the Molluscs originally consisted of isolated calcareous spicules or spines, 

 which were enclosed in a thick cuticle, and projected from the same as in the 

 Prcmeomenia, Ncomcnia, etc. (v. below). 



In Cnjptodiiton the shell is internal, i.e. it is entirely covered by a fold of the 

 integument, which grows over it from all sides. It consists exclusively of the 

 artieulamentum, since the whole dorsal integument is covered by an even cuticle, 

 which therefore forms no tegmentum. 



The only part of a Chituu which can be called the mantle fold is the marginal 

 zone of the body, the ventral side of which encircles the head and foot and forms 

 the lateral boundary of the branchial groove or furrow. Just as the dorsal side of 

 this mantle, which is called 



the zone, carries large " "^^ 



spines, setse, or scales, so 

 may the under surface be 

 covered with small closely- 

 crowded spines. The rest 

 of the integument is bare, 

 being merely covered with 

 a simple epithelium. 



The genus Chitonellus 

 is of great importance in 

 comparing the outer or- 

 ganisation of the Placo- 

 phora with that of the 

 Solenogccstres. The body 

 is not dorso-ventrally flat- 

 tened, as in the Chiton, 

 but nearly cylindrical ; the 

 ventral surface, however, 

 is flattened (Fig. 52), and 

 has a median longitudinal 

 groove. The foot is not 

 externally visible, but can 

 be discovered, much reduced, in the base of the median groove, itself possessing 

 a ventral median groove representing a narrow contracted sole. The flat ventral 

 surface is therefore the mantle. In the narrow cleft on each side, between mantle 

 and foot, in the posterior half of the body, lie the gills, The lateral margin of 

 the body in Chiton is represented in Cliitoarlliis by a mere blunted ridge, which 

 is almost exclusively caused, as may be seen in transverse sections, by a great 

 thickening of the cuticle. 



B. Solenogastres. 



In the Solenogastres (Aplacophora), whose outer organisation has already been 

 sufficiently described (p. 29), the shell is altogether wanting, but the cuticle secreted 

 by the epithelium over the whole body is usually exceedingly thick (Fig. 53). It 

 contains calcareous spicules, which sometimes project above the surface. These, like 

 the spines of the Polyplacojihorn , rise from cellular cups, which are connected with 

 the basal epithelium of the cuticle by nucleated stalks. There can be no doubt 

 that the spicules are form.ed by these cups and nourished by them during growth. 

 The foot, as we have seen, is reduced to a narrow ciliated longitudinal ridge, which 

 rises from the base of the medio-ventral groove. The term mantle is here inappli- 

 cable, except perhaps to the integument which forms tlie lateral boundary of this 

 groove. 



Fio. 52.— Transverse section of Chitonellus, diagrammatic, 

 adapted from figures by Pelseneer and Blumrich. tj, Shell (artieu- 

 lamentum) ; go, gonad ; i, intestine ; a6, vb, branchial arteries and 

 veins ; pc, pleuro-visceral nerves ; x, latero-ventral thickening of 

 tlie cuticle ; jj, foot ; ct, ctenidinm ; pn, pedal nerve ; ft, digestive 

 gland (liver) ; c, secondary coelom ; ao, aorta. 



