Equally abundant are the land Mollusca of the tropics. Prof. C. B. Adams relates that within the limits of a single parish in Jamaica, named Manchester, which measures no more than four miles long and one mile broad, he obtained no fewer than one hundred species. Mr. J. S. Gibbons, in a description of the Mollusca he obtained near St. Ann’s, Curaçao, gives a lively picture of their abundance in an exceptionally favoured locality:—[2]
“Near the outskirts of the town a waste piece of ground supplied me with occupation for all the time I had to spare. Neither grass nor water was to be seen, the only vegetation consisting of a few stunted cacti and still fewer acacia bushes. This, however, was so rich in shells that of several species enough specimens could have been collected in a few yards to supply, I should suppose, all the shell cabinets in the world.... The stones, plants, and ground were covered with Strophia uva L., Tudora megacheila, P. and M., was in equal abundance, suspended by its silk-like thread from acacia boughs, or strewed thickly on the ground underneath. A Bulimulus (B. multilineatus var. sisalensis) abounded on the smaller boughs, while under masses of coral Macroceramus inermis Gundl., Pupa parraiana d’Orb, and P. pellucida Pfr., were abundant. In the loose soil Cylindrella Raveni Bland, Cistula Raveni Bland, and a curious Cionella were so numerous that a spade would have been the best instrument with which to collect them. I wasted a good deal of valuable time in separating them from the soil, when by simply taking away a few handfuls of mould, I might have obtained a larger number of specimens. A species of Stenogyra and a Succinea complete a list, all of which might have been gathered from almost any square yard of ground on the hillside.”
Position of Mollusca in the Animal Kingdom.—Up to very recent times it was usual to regard the Mollusca as one of the four subdivisions of a great family known as Malacozoa, the subdivisions being (1) Mollusca, (2) Tunicata, (3) Brachiopoda, (4) Polyzoa or Bryozoa. This classification is still retained in the leading modern manual on the subject.[3] The progress, however, of investigation leads to the belief that the Mollusca are not so closely related to these other groups as such a classification would seem to imply. The Tunicata, for instance, appear, from the whole course of their development, to occupy a position near to the Vertebrata. The relations of the Brachiopoda and Polyzoa will be more particularly referred to in that part of this History which deals especially with those groups. The position of the Mollusca is, in many respects, one of considerable isolation. Any attempt, therefore, definitely to relate them to one group or another, is, in all probability, to go further than the present state of our knowledge warrants. Especially to be deprecated are systems of classification which confidently derive the Mollusca in general from this or that group. The first undisputed traces of animal life, which appear in the Cambrian epoch, exhibit the same phyletic distinctions as now exist. Sponges, Echinoderms, Mollusca, and Worms, formed already, in those immeasurably remote ages, groups apparently as generally distinct from one another as they are at the present time. It would seem that any theory of development, which confidently teaches the derivation of any one of these groups from any other, is, in the present state of the evidence before us, hazardous in the extreme.
Some indications of relationship, which must not be pushed too far, may be drawn from a consideration of embryonic resemblance. An especial characteristic of the Mollusca is the possession of a particular form of larva, which occurs in one of the stages of development, known as the trochosphere (see p. [130]). This form of larva is shared with two orders of Annelida, the Chaetopoda and the Gephyrea armata, and, in all probability, with the Polyzoa as well. It may also be significant that the adult form in Rotifera bears a close resemblance to the trochosphere larva in those groups.
Basis of Classification.—The Mollusca are divided into four great Orders—Cephalopoda, Gasteropoda, Scaphopoda, and Pelecypoda.[4] Each name, it will be noticed, bears reference to the ‘foot,’ i.e. to the organ of motion which corresponds in function to the foot in the Vertebrata.
In the Cephalopoda the feet, or, as they are more frequently termed, the ‘arms,’ are arranged symmetrically round the head or mouth. The common forms of ‘cuttle-fish’ (Octopus, Loligo) are familiar examples of Cephalopods.
The Gasteropoda crawl on the flat under-surface or ‘sole’ of the foot. Snails, slugs, sea-hares, whelks, periwinkles, and coats-of-mail or chitons are examples of this Order.
The Scaphopoda possess a long tubular shell open at both ends; with their small and elongated foot they are supposed to dig into the mud in which they live. The common Dentalium or tusk-shell of our coasts is a representative of this Order.
Fig. 1.—Examples of the four Orders: A, Cephalopoda; B, Gasteropoda; C, Scaphopoda, and D, Pelecypoda.