a FIG. 140. state of contraction, the soft parts appear very short and puckered up, so that a species of neck is formed between the jaws and the axis; this, however, is not the case in living specimens. All the parts composing the skeleton of the Pedicellariæ exhibit the characteristic reticulated structure of the Echinodermata. The jaws are thin, flattened below, sharp above, and bent nearly at right angles, so C as to form a tooth; the axis is about th of an inch in length and dilated at both extremities, and in shape and structure is very like the spine of an Echinus. On either side of the jaws may be seen a row magnified 130 diameters. of small bicurvate spicula, somewhat resembling those in the disc of the cirrhi of the Echinidæ, but differing from them, as represented at d, in Plate XIV, Fig. 19, of the first volume of the "Histological Catalogue," in having more than one hooked process extending outwards from the point where the curved portion commences. Under a power a, Pedicellaria of Echinus of the Pedicellarice of 4sterias rubens. c, portion of miliaris. b, skeleton of one the skeleton of the same of 40 diameters, as shown at a, in Fig. 109, numerous Pedicellariæ are distinctly visible on the upper dermal surface of Asterias rubens, even after having been dried, but as the soft fleshy stalk is very short and has no calcareous axis, little can be seen except the jaws. Pedicellariæ also exist in the Spatangi, but they are not so evident as in the Echini; the principal varieties found in S. purpureus, according to Forbes, are represented in Fig. 141. FIG. 141. The Pedicellariæ then, without doubt, belong to the animal on which they are found; they are not parasites, but it is difficult to determine their true office; they are probably useful in keeping the shell free from all intruders of a parasitic nature, and may be supposed to perform an analogous function to that of the so-called "Bird's-head processes" of the Bryozoa. Pedicellaria of Spatangus purpureus. the structure of the the Echinodermata, the FIG. 142. Little remains to be said on skeleton of the last class of Vermigrada, which contains 2 Syrinx or Sipunculus nudus (after the order Sipunculidæ ; these animals, except in their internal anatomy, more nearly resemble worms than Echinoderms; they have no cirrhi or tentacula, and no rudiment of a calcareous skeleton, their outer in tegument being either fibrous or covered with spines or hooks. The British species, according to Forbes, are grouped under three distinct families, viz.: Sipunculacea, Priapulacea, and Thalassemacea; in the former of these is included the Syrinx, represented in Fig. 142. The length of the body is stated to be six or eight inches, and the outer rough integument is composed of a tough leather-like substance, having both longitudinal and transverse rugæ. Many of the foreign species have the anterior portion of the body covered with brown nodulated tubercles, or spines, probably of a horny nature. I have examined very many species of Sipunculi, but have as yet entirely failed in making out any trace of a calcareous skeleton, which I believe to be almost universally present in the Echinodermata. LECTURE XV. SKELETON OF MOLLUSCA-BRYOZOA AND TUNICATA. HAVING in my last Lecture finished the description of the structure of the skeleton in the Echinodermata, I next proceed to that of the Mollusca. This subkingdom is now divided into the following classes :— Bryozoa, Tunicata, Conchifera, Gasteropoda, Pteropoda and Cephalopoda, the members of each class differing widely from those of the other classes. In the first, or Bryozoa, the individual animals approximate in character to the Zoophytes, and have until lately been classified with them, all being of microscopic minuteness, whereas among the Cephalopoda, some species are several feet in length; in fact, no one could classify these animals from their external characters alone, but so soon as the investing tunic is removed, traces of high organization are perceptible. In the first class, the Bryozoa, we may have animals either perfectly distinct, or connected by an investing tissue so as to form a colony. When describing the Hydroid Zoophytes, I remarked that the polypes are continuations or developments from the soft animal matter occupying the centre of the horny tubular skeleton, and that the cell in which the polype is contained, is not a part of the body of that polype, but developed before the polype itself. Each polype was also stated to be provided with a series of arms, or tentacula, like the common fresh-water Hydra, for the prehension of food, which is conveyed by them to a globular stomach. Here, on the contrary, the horny or calcareous skeleton is a part of the body of the polype, and although the polype may be injured, the cell will be found to possess the powers of repair. The polype may be solitary, as in Bowerbankia, or several may be collected together in one polypidom, as in Flustra, Lepralia, or in clusters, as in Laguncula, but whatever the shape of the skeleton, the polype contained within it, differs in very many points from that of the true Zoophytes. Thus, for instance, the tentacula are provided with cilia, and are not employed in the prehension of food, this being brought to the mouth by the strong current produced by the vibratile movements of the cilia; the stomach is well developed, and immediately above it, is situated a muscular gizzard, and traces of a liver are also discernible. In Bowerbankia, as shown in Fig. 143, VOL. II. S |