Articulata. —The animals composing this sub-kingdom are bilateral and symmetrical. The abstract pattern or type of an articulate animal, like that of the Vertebrata, is a sym metrical figure.t From this primordial sym metry there are but few deviations, and those exceedingly trifling in amount.
Amongst the Entozoa, the English tape worm (Tcenia solium), if the flat sides be taken as dorsal and ventral, presents an in stance of a-symmetry in the position of its genital orifices, which are situated on the edge of each joint, sometimes on one side, sometimes on the other, indifferently.
In the Natural History series of the Ma seum of the College of Surgeons (No. 205.), there is a portionlof a tmnia lata, which ex hibits a monstrosity very interesting in its bearing on the question of the individuality of half segments (p. 845.). On one side, which for distinction we will call right, there are three half segments, the middle one of which unites with two half segments of the left side, leaving the upper and lower of the three right halves isolated and independent. One of these contains a generative apparatus.
— In this subkingdom absence of symmetry seems to be the rule — its pre sence the exception. There are great prac tical difficulties in the way of finding an ab stract notion or type of a mollusk such as have been found for the Vertebrata and Arti culata. If; however, such type he symme trical, that symmetry is departed from much oftener than it is preserved. There is no appearance in any mollusk of a serial re petition of parts — nothing like serial ho mology, except in the Chitons, whose shell consists of a number of similar symmetrical transverse bands. The highest class of mol lusks, the Cephalopoda, are symmetrical, and when they inhabit shells, as the Argonaut and pearly Nautilus, their shells are symme trical. But the enormous number of species comprised in the classes Gasteropoda and Acephala are nearly all unsymmetrical. The slugs are the most symmetrical of the Gaste ropoda in their external form, but here the air-orifice, for instance, is on one side.
Badiata. — For convenience of illustration, I select from among the animals composing this sub-kingdom the common star-fish. (Figs.
16, 22, &c. Vol. I. Art. ECIIINODERMATA.) This animal presents to our view a flattened form with five exactly similar rays or arms ra diating from the centre, where is posited the mouth. Around the.mouth is a nervous circle consisting of a number of ganglia corresponding to the arms, connected with each other by intercommunicating cords. Each arm is symmetrical in itself. Now this figure may be divided into two symmetrical halves by a line drawn across it in either of five different diameters. What is to decide which of these is to be regarded as the mesial line ? For tunately there is, on the side which is the reverse to that on which the oral orifice is placed, a peculiar spot, the remains of an embryonic structure, which is not in the centre, and will therefore serve for a datum. In certain other echinoderms the anus is situ ated eccentrically, in which case it also may be taken as a datum. But still a line drawn through either of these, the number of the arms not being an even number, will divide one of them in its middle on one side, and pass through the interspace between two of them on the other. This, as the arms are exact repetitions of one another, seems an unnatural and arbitrary proceeding. Still, though it possesses no natural middle line, and consequently is not bilateral, yet is the star-fish a symmetrical animal, for the idea of bilateralism is by no means included in our definition of symmetry.
Whenever a number of exactly similar parts symmetrical in themselves are arranged around a centre, whether their number be two, as in bilateral forms, or five, as in the star-fish, the whole figure is symmetrical. Abandoning then the idea of bilateralism, we may look upon the star-fish as composed of five repe titions arranged around a centre. Regarding it in this manner, we are able to institute a comparison between the form of the star-fish and that of the segmented vertebrates and articulates. The star-fish may be regarded as analogous to one segment of a vertebrate or articulate—an animal with one vertebra.