A guide to the fossil invertebrate animals in the Department of geology and palaeontology in the British museum (Natural history) ... / With 7 plates and 96 textfigures.
- British Museum (Natural History). Department of Geology
- Date:
- 1911
Licence: In copyright
Credit: A guide to the fossil invertebrate animals in the Department of geology and palaeontology in the British museum (Natural history) ... / With 7 plates and 96 textfigures. Source: Wellcome Collection.
150/218 page 124
![Cillery M’liicli may be found creeping on tlie rocks at low tide, has a jointed sliell, and looks like a wood-louse. Examination of the animals that live in these five types of shell shows that they are built on as many plans of structure, and to one or other of tliese plans all molluscs, except the Palaeozoic C(jnularida (p. 141), can easily he referred. Tlierefore the Mollusca are divided into the following five Classes I. AMPHINEURA, of which Chiton is an example, owe their name to the two nerve-cords that run down each side of the body, which is-elongated and symmetrical. The mantle always secretes little plates or spicules of shell-substance. 1 hey are divided into two Orders : (a) Aplacophora, which have no shell other than the spicules, and therefore are not changes of sufficient importance to mention here. II. GASTROPODA, of which the snail Helix is an example, derive their name, meaning Belly-foot, from the position of the large foot beneath the stomach and viscera, which are contained in a hump on the animal’s hack; the surface and folded edges of this hum]) constitute the mantle that forms the shell. Thus the shell is a cone, sometimes short, as in the limpet (Jhtfclla), hut generally long, and coiled either ’in one plane as in the ram’s-horn snail (rianorhis), or spirally as in Helix (Fig. 68, 7). In the common snail it may easily he seen that the edges of the mantle fonn a cavity (the ])allial chamber) on the right side of the animal; and into this open the anus and genital duct, which have been brought towards the mouth end of the body by the curving upwards of the viscera into the hump. In many gastropods this twisting of the end of the gut forwards and to the right side has affected other organs and notably the nerve-cords. This affords a basis for dividing the Gastropoda into two Sub-Classes :— found fossil; (h) Polypla- cophora, which liave a .sliell of eight larger ])ie.ces, sur- rounded by a flexible girdle formed of the mantle-edge, in which are usually smaller ])lates or spicules (Fig. Gfi). Appearing finst in Ordovi- cian rocks, they have per- sisted till the present day, with increasing elaboration of the shell, but with no](https://iiif.wellcomecollection.org/image/b24863841_0150.jp2/full/800%2C/0/default.jpg)
